The Neoproterozoic Quruqtagh Group in eastern Chinese Tianshan: evidence for a post-marinoan glaciation

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1 Precambrian Research 130 (2004) 1 26 The Neoproterozoic Quruqtagh Group in eastern Chinese Tianshan: evidence for a post-marinoan glaciation Shuhai Xiao a,, Huiming Bao b, Haifeng Wang c, Alan J. Kaufman d, Chuanming Zhou c, Guoxiang Li c, Xunlai Yuan c, Hongfei Ling e a Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA b Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, USA c Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing , China d Department of Geology, University of Maryland, College Park, MD 20742, USA e Department of Earth Sciences, Nanjing University, Nanjing , China Accepted 14 October 2003 Abstract The Neoproterozoic Quruqtagh Group in eastern Chinese Tianshan consists of, in ascending order, the Bayisi, Zhaobishan, Altungol, Tereeken, Zhamoketi, Yukkengol, Shuiquan, and Hankalchough Formations. Diamictite units occur in the Bayisi Altungol, Tereeken, and Hankalchough Formations. Our field observations confirm the glacial origin of the Tereeken and Hankalchough diamictites, but the glacial origin of diamictites in the Bayisi and Altungol Formations remain ambiguous. While siliciclastic strata dominate the succession, carbonate units are recognized between the diamictites and immediately atop the Tereeken and Hankalchough diamictites. Carbonates in the Altungol Formation are characterized by highly positive 13 C values (up to versus PDB), comparable to those of pre-marinoan but post-sturtian carbonates. High-resolution sampling of the pinkish cap dolostone (ca. 10 m-thick) overlying the Tereeken diamictite shows a consistent 13 C chemostratigraphic pattern (ca. 5 at the base and slightly decreasing upsection) similar to that of Marinoan cap carbonates. This cap dolostone is also characterized by macropeloids similar to those in the Marinoan-age Raventhroat cap carbonate, Mackenzie Mountains. Chemostratigraphic and sedimentary features are suggestive of a Marinoan age for the Tereeken glaciation. Thus, the younger Hankalchough diamictite may represent a post-marinoan glaciation. This interpretation is consistent with the occurrence of vendotaenid fossils in the Shuiquan Formation. The alternative interpretation the Tereeken and Hankalchough diamictites are, respectively, Sturtian and Marinoan in age is not consistent with the above evidence or the chemostratigraphic features of the Shuiquan Formation and the Hankalchough cap carbonate. A post-marinoan age for the Hankalchough and other glaciogenic deposits indicates that the disappearance of Doushantuo-type acritarchs and the subsequent radiation of macroscopic animals are closely linked, in geochronological terms, with a Neoproterozoic glaciation Elsevier B.V. All rights reserved. Keywords: Neoproterozoic; Glaciation; Carbon isotope; Strontium isotope; Quruqtagh; Tianshan 1. Introduction Corresponding author. Fax: addresses: xiao@vt.edu (S. Xiao), xlyuan@nigpas.ac.cn (X. Yuan). The magnitude, causes, and consequences of Neoproterozoic glaciations have been highly controversial (Hoffman et al., 1998; Hyde et al., 2000; Kennedy /$ see front matter 2003 Elsevier B.V. All rights reserved. doi: /j.precamres

2 2 S. Xiao et al. / Precambrian Research 130 (2004) 1 26 et al., 2001a,b; Condon et al., 2002; Hoffman and Schrag, 2002; Leather et al., 2002). One major barrier is the uncertainty about the number and correlation of these ice ages, which also impinges on our understanding of Neoproterozoic biological evolution and stratigraphic subdivision at a critical time in Earth history. Kaufman et al. (1997), for example, argue that there may have been as many as five Neoproterozoic glaciations recognizable in the reconstructed record of temporal 13 C variations. In contrast, Kennedy et al. (1998) recognize only two Neoproterozoic ice ages the older Sturtian and younger Marinoan episodes, with the latter significantly predating the diversification of Ediacaran animals (Grey et al., 2003). Others have suggested that there are three discrete Neoproterozoic glaciations (Brasier et al., 2000; Knoll, 2000). Previous arguments on the number of Neoproterozoic glaciations critically depend on per- ceived correlation among different basins, as no more than two intervals of glaciogenic deposits... are established in any single Neoproterozoic succession (Kennedy et al., 1998). The Quruqtagh (also spelled as Kuruqtagh or Kuluktagh) Group in the Quruqtagh area (Figs. 1 and 2), eastern Chinese Tianshan, has been cited as one of Neoproterozoic successions that contain three glacial intervals (Brookfield, 1994; Knoll, 2000). But published data (Norin, 1937; Gao and Zhu, 1984; Xu et al., 2003) are insufficient to determine whether all of the three diamictites are glaciogenic and how to correlate them with other Neoproterozoic glaciogenic deposits. Neoproterozoic stratigraphy in eastern Chinese Tianshan was first described by Norin (1937) and later refined by Gao and Zhu (1984), whose stratigraphic terms are followed here (Fig. 3). Previous field investigation recognized multiple diamictites in the Fig. 1. The geographic location of the Chinese Tianshan and the Quruqtagh area. Arrows on the Landsat image of the Quruqtagh area point to the location of measured sections. X: Xishankou section (Figs. 3a and 6a); Y: Yukkengol section (Fig. 7); Y : section near Yukkengol (Figs. 8a and 9a); M: Mochia Khutuk section (Fig. 9b); H: Heishan Zhaobishan section (Figs. 3b, c, 6b, c, 8b, and 9d); H : Hankalchough Peak section (Fig. 9c). Black rectangles correspond to geological maps shown in Fig. 2.

3 S. Xiao et al. / Precambrian Research 130 (2004) Fig. 2. Geological maps of the Xishankou (a) and Heishan Zhaobishan area (b). The Xishankou (A A ; see Fig. 3a for stratigraphic column), Heishan Zhaobishan section (B B and C C ; see Fig. 3b and c for stratigraphic columns), Yukkengol, Mochia Khutuk, and Hankalchough Peak sections are marked on the map. Quruqtagh Group, but Norin (1937) only interpreted those in the Tereeken Formation as glaciogenic. In contrast, Gao and Zhu (1984) interpreted diamictites in the Bayisi, Tereeken, and Hankalchough Formations as glaciomarine in origin. Xu et al. (2003) recently reported 13 C and 18 O chemostratigraphic data of the Quruqtagh Group. Most (85%) of their 13 C measurements, however, were made on siliciclastics (e.g. shales, siltstones, and diamictites). The chemostratigraphic values of such measurements are limited because they record geochemistry of the source materials rather than the ocean waters. But they nonetheless claim that there are three glacial intervals preserved in the Quruqtagh Group. If correct, the Quruqtagh Group represents a Neoproterozoic succession that contains more than two intervals of glaciogenic deposits. In this paper, we present field observations and new chemostratigraphic data of the Quruqtagh Group in order to better understand the regional stratigraphy and to bear on the number of Neoproterozoic glaciations. In our study, we focus our chemostratigraphic analyses on sporadic carbonate beds throughout the succession, particularly cap carbonates overlying two of the three diamictites. The chemostratigraphy of these cap carbonates have not been studied by previous investigators. 2. Methodology Sample preparation and analysis followed the procedures described in the literature (Kaufman et al., 1991; Narbonne et al., 1994; Kaufman and Knoll, 1995). Carbonate samples were cut to make mirroring thin and thick sections. Petrographic observation of thin sections was used to guide microdrilling of corresponding thick sections. Efforts were made, on the basis of petrographic evidence and cathodoluminescence

4 4 S. Xiao et al. / Precambrian Research 130 (2004) 1 26 Fig. 3. Measured sections at Xishankou (a) and Heishan Zhaobishan area (b and c). See Figs. 1 and 2 for section location. examination, to avoid drilling diagenetically altered areas. Micrites and microsparites were sampled from thick sections using a 1 mm drill bit. Some homogenous dolomicrite/dolomicrosparite samples were analyzed using whole rock powders. The powders were then divided into three aliquots for elemental, 13 C and 18 O, and 87 Sr/ 86 Sr analyses. For elemental analysis, 2 4 mg of powder was let react with 1.0 ml 0.5 M acetic acid overnight to dissolve the carbonate but not the silicate minerals. After reaction, 0.1 ml solution was pipetted and diluted to 10.0 ml solution spiked with 1 ppb 115 In. Elemental analyses were performed on a Finnigan Element 2 ICP-MS at Tulane University. Argon plasma forward

5 S. Xiao et al. / Precambrian Research 130 (2004) power was 1250 W and nickel cones (sampler and skimmer) were used during analysis. Analytical precision is better than 5%. Reported data were not corrected for insoluble residue, which is negligible based on visual observation. This may cause inaccuracy of absolute elemental concentrations, but elemental ratios should not be affected. A second aliquot of power was used for 13 C and 18 O determination in two labs. Some samples were measured using a Finnigan MAT 251 mass spectrometer at Nanjing Institute of Soil Science, Chinese Academy of Science. CO 2 was extracted using standard offline technique, with sample reacting with concentrated H 3 PO 4 at 25 C for 12 h. Other samples were analyzed using a Micromass IsoPrime dual-inlet gas source stable isotope mass spectrometer at University of Maryland, which is equipped with a peripheral MultiPrep system for online carbonate reactions. Analytical precision is better than 0.1 for 13 C and 0.3 for 18 O. Samples analyzed in both labs show that inter-laboratory variation is general less than 0.2 for 13 C. 13 C and 18 O data are reported in PDB. A third aliquot of powder (10 30 mg) was leached in 10 ml 0.5 M acetic acid for 87 Sr/ 86 Sr measurement. Dissolution residue was determined so that Sr concentration can also be calculated, independent of elemental analysis described above. Strontium was isolated using standard ion exchange techniques and strontium isotopic compositions were determined on a Finnigan MAT 262 thermal ionization mass spectrometer (TIMS) at Louisiana State University (LSU), Baton Rouge. Several samples were analyzed using a similar method on a Finnigan Triton TIMS at Nanjing University (NU). Multiple measurements of the standard NBS 987 gave an average of ± 9 (2S.E., n = 3; LSU lab) and ± 14 (2S.E., n = 6; NU lab). Reported 87 Sr/ 86 Sr data were not corrected for 87 Rb decay. All analytical results are presented in Table Regional geology The Tianshan is a major orogeny belt in central Asia. It separates the Tarim block to the south from the Kazakhstan and Junggar plates to the north. Paleozoic collisions accreted several microcontinents and island arcs to the Tarim plate (Windley et al., 1990). The Quruqtagh area is located at the southeastern tip of the Tianshan orogeny belt. It is bounded by a major Paleozoic suture to the north and a Cenozoic thrust belt to the south (Windley et al., 1990; Yin et al., 1998). Because of thrusting displacement and the lack of Neoproterozoic outcrops in the Tarim Basin for comparative study, it is difficult to determine whether the Quruqtagh area was part of the Tarim Plate in the Neoproterozoic time. In the Quruqtagh area, the oldest rocks are Paleoproterozoic gneisses and marbles (Bureau of Geology and Mineral Resources of Xinjiang Uygur Autonomous Region, 1982). Unconformably overlying these highly metamorphosed rocks are Mesoproterozoic and Neoproterozoic low-grade metamorphic rocks, mostly phyllites and carbonates, with abundant columnar stromatolites (Zhao et al., 1985). These stromatolitic carbonates are unconformably overlain by thick siliciclastics and diamictites of the Quruqtagh Series (Norin, 1937) or Quruqtagh Group (Gao and Zhu, 1984; Gao et al., 1993). The Quruqtagh Group is succeeded, probably disconformably, by Cambrian Ordovician carbonates (Norin, 1937; Troedsson, 1937; Zhong and Hao, 1990). Upper Paleozoic rocks are restricted and strongly folded because of suturing (Windley et al., 1990). 4. Litho- and biostratigraphy of the Quruqtagh Group and the Lower Cambrian Xishanblaq Formation The Quruqtagh Group is bracketed between earlier Neoproterozoic stromatolitic dolostone and basal Cambrian black chert and phosphorite of the Xishanblaq Formation. It consists of >3 km-thick, predominately siliciclastic sediments, probably representing a Neoproterozoic succession deposited in a rift-drift basin since basalts are common in the lower Quruqtagh Group. It is divided into, in ascending order, the Bayisi, Zhaobishan, Altungol, Tereeken, Zhamoketi, Yukkengol, Shuiquan, and Hankalchough Formations (Fig. 3; Gao and Zhu, 1984). Carbonates occur in the Shuiquan Formation and sporadically in the Altungol, Tereeken, basal Zhamoketi, and uppermost Hankalchough Formations. The carbonate unit in the basal Zhamoketi Formation and atop the Tereeken

6 6 S. Xiao et al. / Precambrian Research 130 (2004) 1 26 Table 1 Geochemical data of Quruqtagh carbonate samples Sample number Stratigraphic height (m) 13 C 18 O 87 Sr/ 86 Sr Sr (ppm) Mn/Sr Hankalchough cap dolostone at Yukkengol (Fig. 9a) YKG YKG YKG YKG YKG YKG YKG YKG YKG Hankalchough cap dolostone at Mochia Khutuk (Fig. 9b) MK ± 5 MK ± 15 MK ± 6 MK ± 6 MK MK MK MK MK MK ± 6 MK MK ± 4 MK MK ± 5 MK MK ± 4 MK ± 4 MK ± 5 Hankalchough cap dolostone at Hankalchough Peak (Fig. 9c) L ± L L-68 Nanjing data ± 5 L ± L-67 Nanjing data ± 4 L L ± L L ± L ± L L ± L Hankalchough cap dolostone at Heishan-Zhaobishan (Fig. 9d) ZBS ± ZBS-6 Nanjing data ± 18 ZBS ZBS-7 Nanjing data ZBS ZBS ± ZBS ±

7 S. Xiao et al. / Precambrian Research 130 (2004) Table 1 (Continued ) Sample number Stratigraphic height (m) 13 C 18 O 87 Sr/ 86 Sr Sr (ppm) Mn/Sr ZBS ± ZBS ± Shuiquan Fm at Yukkengol (Fig. 8a) YKG YKG YKG YKG YKG YKG YKG YKG YKG YKG YKG YKG-83 (duplicate) YKG YKG-82 (duplicate) YKG YKG-81 (duplicate) YKG YKG YKG YKG YKG YKG ± YKG ± YKG YKG ± Shuiquan Fm at Heishan-Zhaobishan (Fig. 8b) ZBS ZBS ± ZBS ZBS ZBS ± ZBS ZBS ZBS ZBS ± ZBS ZBS ZBS ZBS ZBS ZBS ± ZBS ZBS ZBS ± ZBS ZBS ZBS ZBS ZBS ZBS-58 Nanjing data

8 8 S. Xiao et al. / Precambrian Research 130 (2004) 1 26 Table 1 (Continued ) Sample number Stratigraphic height (m) 13 C 18 O 87 Sr/ 86 Sr Sr (ppm) Mn/Sr ZBS ± ZBS-59 Nanjing data ZBS ZBS-61b ZBS-61a ZBS ZBS ZBS ZBS ZBS ± Basal Zhamoketi Fm (Tereeken cap dolostone) at Yukkengol (Fig. 7a) Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta ± 26 Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta ± 19 Delta Delta Delta Delta Delta Delta Delta Delta

9 S. Xiao et al. / Precambrian Research 130 (2004) Table 1 (Continued ) Sample number Stratigraphic height (m) 13 C 18 O 87 Sr/ 86 Sr Sr (ppm) Mn/Sr Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta ± 9 Delta ± 6 Delta-11 (duplicate) ± 4 Delta Basal Zhamoketi Fm (Tereeken cap dolostone) at Yukkengol (Fig. 7b) Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta

10 10 S. Xiao et al. / Precambrian Research 130 (2004) 1 26 Table 1 (Continued ) Sample number Stratigraphic height (m) 13 C 18 O 87 Sr/ 86 Sr Sr (ppm) Mn/Sr Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Basal Zhamoketi Fm (Tereeken cap dolostone) at Yukkengol (Fig. 7c) Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Delta Bedded carbonates in the Tereeken Fm at Heishan-Zhaobishan (Fig. 6c) HS HS HS HS-62b HS-62a HS HS HS HS HS HS-57a HS HS Carbonate laminae in the Tereeken Fm at Heishan-Zhaobishan (Fig. 6b) HS ± HS-40-3m

11 S. Xiao et al. / Precambrian Research 130 (2004) Table 1 (Continued ) Sample number Stratigraphic height (m) 13 C 18 O 87 Sr/ 86 Sr Sr (ppm) Mn/Sr HS-40-2m ± HS-40-1p ± HS ± Carbonates in the Altungol Fm at Xishankou (Fig. 6a) XSK XSK XSK XSK XSK XSK XSK XSK-63a XSK XSK-62a XSK XSK XSK XSK XSK XSK XSK ± XSK XSK XSK ± XSK XSK ± XSK XSK XSK XSK XSK ± XSK Formation is hereafter referred to the Tereeken cap carbonate, and that in the uppermost Hankalchough Formation is referred to the Hankalchough cap carbonate. Most of the Quruqtagh Group was deposited in relatively deepwater slope facies, probably below the fair weather wave base. There is little evidence for shallow-water sedimentation. Instead, turbidite beds occur in the Zhamoketi Formation (Li and Dong, 1991). Only in the Shuiquan is there definitive evidence (stromatolites and microbial laminites) for sedimentation in the photic zone. The Heishan Zhaobishan section (Fig. 3b and c), about 200 km east of the city Korla, is the most complete; here, all formations can be seen in a single, structurally uncompromised section. The Heishan Zhaobishan section was also studied by Gao and Zhu (1984) and Xu et al. (2003). However, the best outcrops of the Bayisi Altungol Formations are in the Xishankou area (Fig. 3a) about 40 km southeast of Korla, and the best outcrops of the Tereeken cap dolostone are in the Yukkengol area about 120 km east of Korla. The Tereeken cap carbonate was mapped by Norin (1937). We measured these sections, as well as three Hankalchough Lower Cambrian sections at Yukkengol, Mochia-khutuk (about 150 km east of Korla), and Hankalchough Peak (about 180 km east of Korla). Location of these sections is marked in Figs. 1 and 2. Multiple sections in the same area allow us to test regional consistency.

12 12 S. Xiao et al. / Precambrian Research 130 (2004) 1 26 Below we summarize the litho- and biostratigraphy of the Quruqtagh Group and the Xishanblaq Formation The Bayisi Formation The Bayisi Formation consists of phyllite-slate grade metasediments and metavolcaniclastics. It is >250 m-thick at Xishankou where four units of greenish gray diamictite (Fig. 4a) are separated by quartz and polylithic metawackes. Quartz, feldspar, chlorite, epidote, zircon, and various mafic minerals are common in the metawackes. Clasts in the metawackes are mm in diameter, angular, and poorly sorted. The thickness of the diamictite units varies from a few meters to more than 100 m. Poorly sorted and rounded cobbles and boulders, many stretched due to metamorphic shearing, occur in silty and muddy matrix of the diamictite (Fig. 4a). Clasts are mostly derived from older metamorphic and igneous rocks, including gneiss, granite, and diabase. Some boulders are perpendicularly or obliquely oriented relative to bedding plane. A ca. 50-m-thick basalt occurs near the top of the Bayisi Formation at both the Xishankou and Heishan Zhaobishan sections. This basalt may provide a radiometric age constraint for the Bayisi Formation. Previous attempt to date this basalt, however, has been unsuccessful; Zhu and Sun (1987) published three widely differing Pb Pb model ages (716, 771, and 664 Ma) for the Bayisi basalt at Xishankou. The Bayisi diamictite units were interpreted as glaciomarine in origin by Gao and Zhu (1984). This interpretation may well be correct, given the regional occurrence of these thick, poorly sorted diamictite units that sometimes contain obliquely oriented clasts. However, unequivocal evidence for glaciation, such as glacial striations on boulders, is lacking, and may have been obliterated by later deformation and metamorphism. In the study area, the Bayisi diamictites are not immediately followed by a cap carbonate. We note that cap carbonates can be discontinuous in deep-water slope facies (cf. Myrow and Kaufman, 1999) and that the Sturtian cap carbonate is typically poorly developed (e.g. in South China). Because of metamorphism, imprecise radiometric ages, and the lack of a cap carbonate, neither the glaciomarine nature nor the age of the Bayisi diamictite units can be unambiguously determined The Zhaobishan Formation The overlying Zhaobishan Formation is about 300 m-thick. It consists of metawackes, metarenites, calcareous siltstones, and slates. Quartz, feldspar, and lithic fragments ( mm in diameter) are common. Mafic igneous fragments are still present, although not as common as in the Bayisi Formation. Sedimentary maturity (rounding and sorting) is also better than the Bayisi sediments. No diamictites occur in the Zhaobishan Formation The Altungol Formation The succeeding ca. 300 m strata of the Altungol Formation consist predominately of metamorphosed mudstones, siltstones, and silty arenites, with several horizons of diamictites (typically <50 m in thickness). According to Gao and Zhu (1984), the Altungol Formation begins with a diamictite of unstable thickness. This diamictite unit is less than 20 m-thick in the Heishan Zhaobishan area. Altungol diamictites, like the Bayisi diamictites, are metamorphosed and deformed. The diamictite beds are typically thinner than massive diamictite beds (several hundred meters thick) of the Tereeken Formation. Although the Altungol Formation is mostly siliciclastic, bedded dolostones occur over a 50-m interval in this formation at the Xishankou section. Gao and Zhu (1984) suggested that the Altungol diamictite beds have a closer affinity with the overlying Tereeken diamictites and they argue that the Altungol and Tereeken jointly represent a mid-quruqtagh ice age. Our field observation suggests that the Altungol diamictites are actually more similar to those in the Bayisi Formation in clast composition, degree of metamorphism, and thickness of individual diamictite beds. Unlike the overlying Tereeken diamictites, no striated clasts have been unambiguously identified in the Altungol diamictites. If all these diamictites are glaciogenic (Gao and Zhu, 1984), it is more likely that the Altungol and Bayisi diamictites represent pulses of a single ice age, separated from the Tereeken ice age by a tectonic event. But it is still possible that neither the Bayisi nor the Altungol diamictites are glaciogenic.

13 S. Xiao et al. / Precambrian Research 130 (2004) Fig. 4. (a) Bayisi diamictites, Xishankou section. Rock hammer head 17 cm. (b) Striated boulders in the Tereeken diamictite, Heishan Zhaobishan section. (c) Sharp contact (dashed line) between Tereeken diamictite (lower right) and the capping dolostone in the basal Zhamoketi Formation (upper left), Yukkengol section. The cap dolostone (bracket) is about 10 m-thick. (d) Close up of the contact (dashed line), Yukkengol section. Cap lense (circled) 5 cm in diameter. (e) Photomicrograph of macropeloids in the Tereeken dolostone, Yukkengol section. (f) Close up of macropeloids showing constituent peloids. (g) Repeated sandstone/siltstone cycles in the Zhamoketi Formation, Heishan Zhaobishan section. View about 200 m wide. (h) Large boulders in the Hankalchough diamictite, Heishan Zhaobishan section.

14 14 S. Xiao et al. / Precambrian Research 130 (2004) The Tereeken Formation The Tereeken Formation, about 2000 m-thick, is best exposed in the Heishan Zhaobishan area and the Yukkengol area. It is characterized by at least five massive and little metamorphosed diamictite units, with subordinate siltstones and rare but important carbonate units. These diamictite units are each several tens to hundreds of meter in thickness, and they are separated by finely laminated silty rhythmites. The diamictites are ubiquitously covered by varnish in the Heishan Zhaobishan area, but their fresh color is dark gray to greenish gray. Unambiguous glaciogenic features, such as striated clasts (Fig. 4b) and dropstones, are common throughout the diamictite units. Clasts in diamictites are derived from Paleoproterozoic gneiss, granite, and marble. Several thin carbonate laminae (<10 mm-thick and laterally continuous for more than 100 m) occur within the silty rhythmites in the lower Tereeken Formation at Heishan Zhaobishan. These laminae consist of vertically oriented, upward-growing calcite crystals, and are typically inundated by overlying siltstone. In addition, a bedded carbonate unit occurs in the lower Tereeken Formation, between two massive diamictite beds, at Heishan Zhaobishan. It is 12 m-thick and consists of homogeneous dolomicrites and dolomicrosparites. Unlike the metamorphosed diamictite units in the Bayisi and Altungol Formations, those in the Tereeken Formation are undoubtedly glaciogenic. That there exist laminated siltstones and bedded dolostones between Tereeken diamictite units indicates that there was an active hydrological system during the deposition of the Tereeken Formation (cf. Condon et al., 2002) The Zhamoketi Formation A 10-m-thick dolostone unit atop of the Tereeken Formation marks the beginning of the Zhamoketi Formation (Fig. 4c and d). This unit is best seen in the Yukkengol area. It was recognized as the Yukkengol limestone by Norin and was used by him as a convenient lithostratigraphic marker to separate the Tereeken Formation and overlying sediments (Norin, 1937). Notably, this cap dolostone is not well developed in the Heishan Zhaobishan area; here, decimeter-sized carbonate nodules occur in shales ca. 3.5 m above the Tereeken diamictite. This carbonate unit consists of thin-bedded dolostone that is dark grey on fresh outcrops but distinctively pink-colored on weathered surface. It can be divided into three subunits. The lower subunit, about m-thick, consists of homogeneous calcitic dolomicrosparite, with a few thin shaly beds. The middle subunit, m-thick, consists of macropeloidal dolomite (Fig. 4e; cf. James et al., 2001). The macropeloids, between 0.2 and 4.0 mm but on average 0.5 mm in diameter, are made of smaller peloids about 0.05 mm in size. They sometimes but not always form inversely graded beds each about several millimeters in thickness. The top of these macropeloidal beds is typically truncated by a veneer of micrite. The macropeloids are dolomitized but usually cemented by blocky calcite (Fig. 4f). The upper subunit, about 6 m-thick, consists of homogeneous dolomicrosparite. Occasional limestone and dolostone interbeds occur in overlying shaly siltstones. We interpret this 10 m carbonate unit as the cap carbonate associated with the Tereeken glaciation. Its pinkish color and macropeloidal structures are reminiscent of Marinoan cap dolostones (e.g. the Ravensthroat Formation in the Mackenzie Mountains, northwestern Canada; James et al., 2001). The Tereeken cap dolostone in the Yukkengol area notably lacks such Marinoan features as Tepee-like structures and an upper limestone unit with crystal fans. Tepee-like structures and crystal fans, however, may be facies-dependent structures. Indeed, these structures are not ubiquitously present in all Marinoan cap carbonates. We tentatively interpret the basal Zhamoketi carbonate unit as a Marinoan cap and will test this hypothesis using chemostratigraphic data. The rest of the Zhamoketi Formation consists of ca. 800 m of meter-scale cycles of sandstone and siltstone (Fig. 4g). Typically, a cycle begins with a normally graded fine conglomerate or coarse sandstone, followed by parallel laminae of siltstone. Occasionally, a shaly unit may develop at the top of the cycle, and a siltstone unit with small-scale cross-lamination or convolute lamination may occur above the graded bed. Zhamoketi sandstones have relatively low sedimentary maturity. They typically contain 70% quartz,

15 S. Xiao et al. / Precambrian Research 130 (2004) % feldspar, 10% lithic fragments, and 10% matrix and cement. Detrital zircons have been seen in some thin sections. Millimeter-sized pyrite molds occur on outcrop. Some sandstones bear sole markings such as swarmed flute casts, groove casts, and load casts. These sandstone siltstone cycles have been interpreted as Bouma sequences deposited in a continental slope (Li and Dong, 1991). Li and Dong further suggested that sole markings and spatial patterns of grain size and bed thickness indicate that sedimentary source came from the west and south The Yukkengol Formation The Yukkengol Formation is separated from the underlying Zhamoketi Formation by an 80-m-thick diabase sill (Gao and Zhu, 1984). It is problematic to use intrusive igneous rock as stratigraphic marker, but this sill seems to be regionally consistent in the Heishan Zhaobishan area. The Yukkengol Formation is generally finer than the underlying Zhamoketi Formation. It consists of ca. 200 m of finer siltstones and shales. A few shale samples contain strongly carbonized sphaeromorphic and filamentous microfossils. The biostratigraphic significance of these microfossils is limited by their simple morphology and poor preservation The Shuiquan Formation The Yukkengol Formation passes upward to buffcolored dolostone of the Shuiquan Formation. The Shuiquan dolostone is about 22 m-thick near Yukkengol but over 70 m-thick in the Heishan Zhaobishan area. Centimeter-scale ribbonites, stromatolites, and microbially laminated dolomicrosparites are present in the Shuiquan Formation, suggesting sedimentation in the euphotic zone. Gao et al. (1980) and Gao and Zhu (1984) reported carbonaceous fossils filaments about 2 mm wide and up to 40 mm-long from the Shuiquan Formation. The illustrated fossils (Gao et al., 1980, Plate XI, Fig. 25) appear to be authentic and can be reasonably interpreted as a vendotaenid fossils. Vendotaenid fossils commonly occur in post-marinoan rocks; if so, these fossils would preclude a Marinoan age for the overlying Hankalchough Formation The Hankalchough Formation The Hankalchough Formation consists of ca. 400 m (thickness measured in the Heishan Zhaobishan area) of light gray diamictite. A dolostone unit (1 5-m-thick) marks the end of the Hankalchough Formation. Clasts in the diamictite are derived from ancient quartzite, gneiss, diabase, and carbonate. Some carbonate boulders contain overturned stromatolites, probably derived from older stromatolitic dolostones in the Paergangtagh Group that underlies the Quruqtagh Group (Zhao et al., 1985). Abundant dropstones occur in the Hankalchough diamictite (Fig. 4h). At Mochia Khutuk, fresh outcrop exposed by mining trenches the overlying phosphorites of the Xishanblaq Formation were mined in the 1970s shows that at least the uppermost few meters of Hankalchough diamictite and rhythmites are rich in organic matter and pyrite. The Hankalchough diamictite is glaciomarine in origin an interpretation supported by the occurrence of dropstones. The occurrence of disseminated pyrite and organic matter in the uppermost Hankalchough Formation suggests active primary bioproductivity during or in the wake of the Hankalchough glaciation. Norin (1937) reported abundant indisputable organic remains, probably foraminifera, from carbonate boulders in the Hankalchough diamictites a claim that would have important biostratigraphic significance. Unfortunately, Norin s claim has not been confirmed by our field observation or by other geologists worked in this area (Gao and Zhu, 1984). In the four sections that we examined, a 1 5 m bed of muddy-silty dolomicrite occurs in the uppermost Hankalchough Formation, capping the organicand pyrite-rich diamictites. This cap dolostone is best seen in mining trenches at Mochia Khutuk. The transition from diamictite to dolomicrite is gradual, with upsection increase in carbonate and concurrent decrease in ice-rafted debris. Disseminated pyrite grains (5 10 m in size) are common in the cap dolostone. This dolomicritic unit is different from typical Marinoan or Sturtian cap carbonates that usually have a sharp contact with underlying diamictites and very little siliciclastic component.

16 16 S. Xiao et al. / Precambrian Research 130 (2004) The basal Cambrian Xishanblaq Formation The Hankalchough Formation is overlain by several tens of meters of black cherts and phosphorites of the Xishanblaq Formation. The contact is probably disconformable. Although no incision valleys can be seen between the two formations, a 10-cm-thick reddish clay layer at the Mochia Khutuk section may represent a paleosol separating the Quruqtagh Group and the Xishanblaq Formation. In the Heishan Zhaobishan area, the Xishanblaq cherts and phosphorites are intercalated with thick igneous rocks. In the Yukkengol area, the Xishanblaq Formation contains the septate tubular fossil Megathrix longus (Fig. 5a), small shelly fossils such as Kaiyangites novilis (Fig. 5b), and abundant Michrystridium-like acritarchs (Fig. 5c). These microfossils consistently occur in pre-trilobite Cambrian (or Meishucunian) cherts and phosphorites in South China (Qian and Yin, 1984; Yin, 1995). A Meishucunian ash bed in South China has been dated from ± 1.5 (σ) Ma (Jenkins et al., 2002), putting an upper age constraint on the Hankalchough glaciation. 5. Chemostratigraphy of Quruqtagh carbonates 5.1. The Altungol Formation, Xishankou section (Fig. 6a) At Xishankou, dolostones occur in a 50-m-thick interval in the Altungol Formation. 13 C values of these dolostones are highly positive (up to ), while 18 O values are extremely negative (ca. 16 ). Strontium concentration is between 400 and 1000 ppm, and 87 Sr/ 86 Sr ratios of four measured specimens are between and The Tereeken Formation, Heishan Zhaobishan section (Fig. 6b and c) Calcite laminae of a few millimeters in thickness and a 12-m-thick bedded dolomicrite unit occur in the Tereeken Formation at Heishan Zhaobishan. Both calcite laminae and the dolomicrite unit have consistently negative 13 C values, between 4 and 6, and highly depleted 18 O compositions (ca. 16 ) similar to those from the underlying Altungol Formation. Strontium concentration of bedded dolomicrites is typically >2000 ppm, and that of calcite laminae is between 300 and 1200 ppm. 87 Sr/ 86 Sr ratios of the calcite laminae are very radiogenic, between and The Tereeken cap dolostone in the basal Zhamoketi Formation, Yukkengol section (Fig. 7) Decimeter-scale sampling of the Tereeken cap dolostone at three closely spaced sections at Yukkengol shows that 13 C profiles of the cap dolostone start from less than 5 at the base, rise to between 4 and 5 within the first 10 cm, stabilize at this value for most of the unit, and then migrate toward 6 to 7 near the top of the cap dolostone. 18 Oof the cap dolostone is mostly between 9 and 12. One sample of the carbonate nodules near the base of the Zhamoketi Formation at Heishan Zhaobishan has been analyzed and show similar 13 C( 7 ) but lighter 18 O values ( 16 ). Strontium concentration of this sample is 346 ppm. 87 Sr/ 86 Sr ratios of five measured samples are also very radiogenic, ranging from to The Shuiquan Formation, Yukkengol and Heishan Zhaobishan sections (Fig. 8) The 13 C profile of the Shuiquan Formation at Yukkengol shows a positive shift from 3 to +3, and the 18 O profile also shows a positive trend from less than 12 near the bottom to 1 near the top of the formation (Fig. 8a). At Heishan Zhaobishan (ca. 80 km east of Yukkengol), however, the 13 C profile displays a much greater positive shift from 10 to near 0, and the 18 O evolves from 14 to 7, over some 75 m of section (Fig. 8b). Strontium concentration of the Shuiquan dolostone at both sections ranges from 130 to 1300 ppm. 87 Sr/ 86 Sr ratios of 10 specimens are between and Gao et al. (1993) reported similar 87 Sr/ 86 Sr data ( , uncertainty not given) for a single sample of the Shuiquan carbonate in the Heishan Zhaobishan area The Hankalchough Formation (Fig. 9) The Hankalchough diamictite is capped by a 1 5 m bed of muddy-silty dolomicrite at the Yukkengol,

17 S. Xiao et al. / Precambrian Research 130 (2004) Fig. 5. Photomicrographs of microfossils from the basal Cambrian Xishanblaq Formation, Yukkengol section: (a) Megathrix longus, (b) Kaiyangites novilis, (c) Michrystridium-like acritarchs. Fig C and 18 O profiles of carbonate units in the Yukkengol and Tereeken Formations. Master stratigraphic column is shown in the left; only the predominate lithology is shown for each formation. Approximate stratigraphic positions of 13 C and 18 O profiles are indicated. Also indicated are approximate stratigraphic positions of 13 C and 18 O profiles shown in Figs (a) Altungol carbonate, Xishankou section. (b) Carbonate laminae in the Tereeken Formation, Heishan Zhaobishan section. (c) Bedded carbonates in the Tereeken Formation, Heishan Zhaobishan section.

18 18 S. Xiao et al. / Precambrian Research 130 (2004) 1 26 Fig C and 18 O profiles of the Tereeken cap dolostone in the basal Zhamoketi Formation, Yukkengol section. Parts (a) and (b) are 5 km apart, and (b) and (c) are 50 m apart. Note stratigraphic position of macropeloidal dolostone. Gray shading represents stratigraphic equivalency. See Fig. 6 for lithology legends. Mochia Khutuk, Hankalchough Peak, and Heishan Zhaobishan sections. Isotopic compositions of this dolomicrite unit are highly variable across the basin but stratigraphically consistent within two of the four measured sections (Fig. 9b and d). At Yukkengol, only four specimens were measured and the 13 C values are between 0 and 4 (Fig. 9a). At Mochia Khutuk, 13 C and 18 O values range between 4 and 6, and between 3 and 6, respectively (Fig. 9b). 13 C values of the Hankalchough Peak and Heishan Zhaobishan (Fig. 9c and d) sections, however, are remarkably low ( 12 to 16 ) but 18 O values are normal ( 2 to 5 at Hankalchough Peak and 0 to 2 at Heishan Zhaobishan). These data suggest a spatial gradient: the two sections in the west (Mochia Khutuk and Yukkengol) are significantly more enriched in 13 C than the two sections in the east (Heishan Zhaobishan and Hankalchough Peak). There seems to be a similar spatial gradient in 87 Sr/ 86 Sr too: the western sections ( ) are less radiogenic than the eastern sections ( ) although all 87 Sr/ 86 Sr ratios are significantly greater than typical Neoproterozoic carbonates. Strontium concentration of the Hankalchough cap carbonate is generally low, between 100 and 500 ppm. Fig C and 18 O profiles of the Shuiquan Formation at the Yukkengol section (a) and Heishan Zhaobishan section (b). Gray shading represents stratigraphic equivalency. See Fig. 6 for lithology legends.

19 S. Xiao et al. / Precambrian Research 130 (2004) Fig C and 18 O profiles of the Hankalchough cap dolostone at the Yukkengol section (a and a ), Mochia Khutuk section (b and b ), Hankalchough Peak section (c and c ), and Heishan Zhaobishan section (d and d ). 13 C and 18 O are plotted in separate panels to avoid overlapping. See Fig. 6 for lithology legends. 6. Discussion 6.1. Diagenetic alteration of δ 13 C values The Quruqtagh Series has probably undergone some degree of diagenetic alteration. This is evident from its tectonic position in a Paleozoic orogeny belt, its lower metamorphic grade (particularly in the lower Quruqtagh Group), and the very radiogenic Sr isotopes. Since most diagenetic processes tend to reduce sedimentary 13 C values, the default interpretation is that Quruqtagh 13 C data represent minimum estimate of sedimentary values and therefore positive 13 Cexcursions should have more chemostratigraphic significance than negative ones. Below we evaluate possible diagenetic alteration of Quruqtagh 13 C values based on petrographic features, trace element concentrations, 18 O, and 87 Sr/ 86 Sr values. When pooled together, the 13 C 18 O, 13 C Sr, and 13 C Mn/Sr crossplots of the carbonate units in the Quruqtagh Group do not follow a diagenetic trend (Fig. 10a c), while the 13 C 87 Sr/ 86 Sr crossplot does show a negative correlation (Fig. 10d). This indicates non-uniform diagenetic alteration and/or stratigraphic variation of 13 C values. To further investigate these possibilities, we consider the geochemical signatures of each carbonate unit separately (Figs. 10b d and 11). Dolostones of the Altungol Formation at Xishankou show relatively smaller variation in 13 C than in 18 O(Figs. 6a and 11a). 18 O values are extremely negative (most around 16 ), Sr concentrations below 1000 ppm, and 87 Sr/ 86 Sr ratios are the least radiogenic in the Quruqtagh Group. Considering the slate-phyllite metamorphic grade of the Altungol Formation, it is interesting to note that this formation has the highest 13 C values. We interpret that the Altungol 13 C values were somewhat diagenetically reset, and the maximum 13 C value of is likely a minimum estimate of depositional 13 C values. Carbonates of the Tereeken Formation at Heishan Zhaobishan have consistently negative 13 C values (between 4 and 6 ; Figs. 6b, c and 11a). They

20 20 S. Xiao et al. / Precambrian Research 130 (2004) 1 26 Fig C 18 O (a), 13 C Sr (b), 13 C Mn/Sr (c), and 13 C 87 Sr/ 86 Sr (d) crossplots of pooled data. Carbonate units are differentiated by symbols shown in (b), which also apply to (c) and (d). Corresponding 13 C and 18 O profiles are indicated in inset. have highly depleted 18 O compositions, relatively high Sr concentrations, low Mn/Sr ratios (with one exception), and more radiogenic 87 Sr/ 86 Sr ratios (compared to the Altungol Formation). Either the 13 C values of these fine-grained carbonates are completely reset, or this unit records negative carbon isotopic composition of seawater alkalinity during a Neoproterozoic glaciation (cf. Corsetti et al., 2003; contra Kennedy et al., 2001a). The Tereeken cap dolostone of the basal Zhamoketi Formation in the Yukkengol area has a narrow range of 13 C values but highly variable 18 O values (Figs. 7 and 11b), consistent with greater buffering capacity of 13 C than 18 O. The dolostone is composed of homogenous microsparites and macropeloids. The stratigraphic and regional consistency of 13 C values measured at three sections in the Yukkengol area seems to suggest that the Tereeken cap dolostone may record sedimentary 13 C values, despite radiogenic 87 Sr/ 86 Sr ratios. The diagenetic history of the Shuiquan Formation and the Hankalchough cap carbonate is more complicated. 13 C profiles of both carbonate units show considerable spatial variation, and the Shuiquan Formation has the largest stratigraphic 13 C variation in the Quruqtagh Group. 13 C values of equivalent strata are generally greater in the western than eastern Quruqtagh. For example, the Shuiquan Formation at Yukkengol has an average 13 C value of 0.2, compared with 6.4 at Heishan Zhaobishan (Fig. 11c).

21 S. Xiao et al. / Precambrian Research 130 (2004) Fig C 18 O crossplot of the Altungol and Tereeken Formations (a), the Tereeken cap dolostone (b), the Shuiquan Formation (c), and the Hankalchough cap dolostone (d). Considerable spatial variation is obvious in (c) and (d), where 13 C values of eastern Quruqtagh sections tend to be lower than those of western Quruqtagh sections. Sections are differentiated by symbols and (in c and d) delineated by dashed lines. Average 13 C values of each section are marked on (c) and (d). Corresponding 13 C and 18 O profiles are indicated in insets. Similarly, average 13 C values of the Hankalchough cap dolostone shift from 1.7 at Yukkengol, 5.4 at Mochia Khutuk, 13.6 at Hankalchough Peak, to 16.5 at Heishan Zhaobishan (Fig. 11d). The spatial 13 C disparity of supposedly equivalent carbonate units can be interpreted in different ways: (1) variable post-depositional alteration, (2) diachroneity of carbonate deposition across the basin because of transgressive onlapping from the eastern to western Quruqtagh, or (3) significant intra-basinal 13 C heterogeneity (perhaps partly recording a 13 C depth gradient insofar as the eastern Quruqtagh is more offshore than the western Quruqtagh; Li and Dong, 1991). Distinguishing these alternative possibilities is difficult, because it is possible that a combination of all three factors contributed to the observed spatial pattern. The 13 C 18 O and 13 C Sr crossplots of the Shuiquan Formation seem to follow a diagenetic trend (Figs. 10b and 11c). Diachroneity due to westward onlapping implies that the Shuiquan carbonate at Yukkengol (Fig. 8a) only correlates to the upper Shuiquan Formation at Heishan Zhaobishan (Fig. 8b); this is supported by the magnitude of 13 C shift as well as regional stratigraphy (Li and Dong, 1991). The carbon isotopic difference (ca. 6 ) between the two sections (Fig. 8) is too great to be entirely attributed to depth