Chapter 4 Genesis and tectonic significance of basic intrusion in the Xilin Gol Complex

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1 Chapter 4 Genesis and tectonic significance of basic intrusion in the Xilin Gol Complex The Solonker suture zone, extending from Solonker, via Sonidyouqi, to Linxi, has been widely recognized to record the terminal evolution of the CAOB in Inner Mongolia (Tang, 1990; Sengör et al., 1993; Xiao et al., 2003). The occurrence of bimodal magmatism in the Xilinhot area emplaced at 280 Ma suggests an extensional setting; it is interpreted to reflect the final amalgamation of arc-related terranes in the Solonker zone by the Late Carboniferous (Zhang et al., 2008). However, many lines of evidence indicate that the final collision time of the Solonker zone was between the Late Permian and earliest Triassic (Xiao et al., 2003; Miao et al., 2008; Li et al., 2008; 2009). The variations on final collision time partly reflect the lack of isotopic ages and different interpretations of the geological environments of key tectonic units. The Xilin Gol Complex (also called Xilinhot Complex), located near the Solonker suture of the southernmost CAOB (Tang, 1990; Shi et al., 2003), is an Middle-Late Proterozoic terrane that underwent strong alteration during the northern subduction of the Paleo-Asian Oceanic crust at 452 Ma (Li et al., In press). The youngest age group is represented by a SHRIMP U-Pb zircon age of 341 Ma (Shi et al., 2003) and a LA-ICPMS U-Pb zircon age of 340 Ma (Li et al., In press) from biotite-plagioclase gneiss in the Xilin Gol Complex. Because of the high closure temperature of ~900 C for the Pb-U-Th isotopic system in zircon (Lee et al., 1997), these zircon ages may not reflect the last tectonometamorphic event recorded by the complex, which may be related to the final collision of the CAOB. This chapter aims to put age constraints on the final collision time of the CAOB by careful interpretation of different isotopic ages from different parts of the Xilin Gol Complex. To exactly constrain the timing of this last tectonometamorphic event, we date biotite from biotite-plagioclase gneiss (803-11) and hornblende from plagioclase-amphibolite (803-8) in the Xilin Gol Complex by the 40 Ar- 39 Ar technique. The protolith of the plagioclase- amphibolite was interpreted to be a basic intrusion into the biotite-plagioclase gneiss. The emplacement of the protolith of the plagioclase-amphibolite could be after the main deformation event related to the Early Paleozoic oceanic subduction and another 59

2 Genesis and tectonic significance of basic intrusion in the Xilin Gol Complex amphibolite-facies metamorphism after the emplacement occurred, which may be related to the final collision of CAOB. Here, we present a study of the petrology, geochemistry and LA-ICPMS U-Pb zircon ages of the plagioclase-amphibolite to constrain the tectonic evolution of the CAOB after Early Paleozoic subduction, including the exact timing of final collision between the Southern Mongolian active margin and the North China Craton. 4.1 Geological setting The eastern section of the CAOB was divided into three parts (see Fig.1-2): the southern accretionary zone between North China Craton and the Solonker suture zone, the Solonker suture zone itself and the northern accretionary zone between the South Mongolia microcontinent (Xiao et al., 2003). The southern accretionary zone is characterized by the Middle Ordovician to Early Silurian Ondor Sum subduction-accretion complex and the Bainaimiao arc. The northern accretionary zone extends southward from a continental margin that was active during Devonian to Carboniferous times, through the Hegenshan ophiolitic accretionary complex to the Late Carboniferous Baolidao arc. Complete subduction of the Paleo-Asian Ocean caused the two opposing active continental margins to collide, leading to formation of the Solonker suture (Xiao et al., 2003). The study area is located in central Inner Mongolia around the Xilinhot City and the Linxi County. It is divided into two Paleozoic tectonic units separated by the Xar Moron fault: the Tuchengzi Early Paleozoic tectonic belt, which is the eastwards extension of the Ondor Sum subduction-accretion complex, and the Linxi-Xilinhot Late Paleozoic-Early Triassic tectonic belt (Fig. 4-1). The latter can in turn be divided into 3 parts: the Xar Moron fault belt, the Shangde Ardg anticlinorium that is equivalent to the Baolidao arc-accretion complex, and the Linxi synclinorium that is equivalent to the Solonker suture zone. The Baolidao arc-accretion complex contains arc volcanic rocks and accretionary wedges. The arc is composed chiefly of variably deformed metaluminous to weakly peraluminous, hornblende-bearing gabbroic diorite, quartz diorite, tonalite and granodiorite, and contemporaneous volcanic rocks have geochemical data suggesting formation in island arc and back-arc settings (Chen et al. 2000; Xiao et al., 2003). The southern margin of the Baolidao arc is abundant in faults, folds and unconformable surface. An EW-NEE oriented Early Paleozoic reverse fault and a NE oriented Late Paleozoic-Early Triassic reverse fault were distinguished in this area (Fig. 4-2; GS-CUG, 2008). 60

3 Chapter 4 The Xilin Gol Complex occurs within the southern margin of the Baolidao arc accretionary zone and is distributed discontinuously as variably sized tectonic blocks between Xilinhot and Baiyinchagan (Fig. 4-2) covering a total area of about 600 km 2. The complex is in fault contact with the upper Silurian sedimentary strata that formed in a shelf environment and is locally covered by Mesozoic vocanosedimentary with angular unconformity. Many Late Paleozoic to Mesozoic granites and numerous later granitic and quartz veins intruded into the complex (Fig. 4-2, section A-B). The Xilin Gol Complex is composed mainly of gneisses, schists and amphibolites, most of which have undergone the same deformation and dynamic metamorphism. Gneisses are the most common rocks, including widespread biotite-plagioclase gneiss, as well as amphibole-plagioclase gneiss and granitoid gneiss. Lenticular or quasi-lamellar plagioclase-amphibolites are intercalated in biotite-plagioclase gneiss without apparent deformation. Fig. 4-1 Tectonic frame of the research area in the Xilinhot-Linxi area (GS-CUG, 2008) 61

4 Genesis and tectonic significance of basic intrusion in the Xilin Gol Complex Fig. 4-2 Geological map of Xilinhot area showing the distribution of Xilin Gol Complex and the cross section of Xilin Gol Complex (GS-CUG, 2008). 62

5 Chapter Petrography, mineral chemistry and thermobarometry The plagioclase-amphibolite has fine grained equigranular texture and massive structure. It is mainly composed of plagioclase (50%), hornblende (45%) and biotite (5%). Accessory minerals are zircon, apatite and magnetite. Local metamorphic differentiation is obvious from the presence of many remelting felsic veins. From the mineral assemblage, we assume that the rock was subjected to amphibolite facies metamorphism. Samples and -8 were chosen for electron microprobe analyses of plagioclase and hornblende, one of which (803-6) had developed a remelting vein. The results are listed in Tables 4-1 and 4-2. Plagioclase in both samples is andesine, however, plagioclase from sample has higher anorthite content (52-59%) than those of sample (An ). All the hornblendes in both samples are calcic amphibole ((Ca+Na) B 1.34 and Na B <0.67) by the classification schema of hornblende (Leake, 1978). Hornblende from sample has higher content of Fe 2+, Ca 2+, Al Ⅵ and lower Na +, Mg 2+, Fe 3+ than sample Given the presence of remelting vein in 803-6, the differences between the two samples may be the result of the metamorphic differentiation of remelting vein, which indicates that partial melting occurred during metamorphism. Hornblende geobarometry using the calibrations of Hammartron and Zen (1986) and Hollister et al. (1987), results in pressures of formation of hornblende of GPa and GPa, respectively, which are in excellent agreement. According to the hornblende-plagioclase geothermometer of Blundy and Holland (1990) the metamorphic temperature was C. Table 4-1 Electronic microprobe results (wt%) of plagioclases from plagioclase-amphibolites and related parameters No SiO TiO Al 2 O FeO MnO MgO CaO Na 2 O

6 Genesis and tectonic significance of basic intrusion in the Xilin Gol Complex K 2 O Cr 2 O An Ab Or Species andesite andesite Number of ions on basis of 8 oxygen atoms. Table 4-2 Electronic microprobe results (wt%) of hornblendes from plagioclase-amphibolites and related parameters No SiO TiO Al 2 O FeO* MnO MgO CaO Na 2 O K 2 O Cr 2 O Fe Fe Ca Na Al Ⅳ Al Ⅵ Species calcic amphibole calcic amphibole Numbers of ions were calculated based on the sum of cations both in tetrahedron and octahedron FM=13 (Si+Al Ⅳ =8, Al Ⅵ +Fe 3+ +Fe 2+ +Ti+Mg+Mn=5) and electrovalence balance (Jarrar, 1998). 64

7 Chapter Whole-rock geochemistry Major and trace element compositions of 6 plagioclase-amphibolite samples from the study area are listed in Table 4-3. The plagioclase-amphibolite samples are metabasites, characterized by low SiO 2 ( %) and alkalis (Na 2 O+K 2 O= ), and high FeO T ( %), MgO ( %), Al 2 O 3 ( %) and CaO ( %). The Mg # [Mg/(Mg+Fe 2+ )] is with an average of 0.60, which is lower than expected for a primary magma ( ). The ratio of Na 2 O/K 2 O is reflecting high Na 2 O ( %) and low K 2 O ( %), and the Rittmann indices ( ) are less than 3.3, indicating the magma belongs to sodium sub-alkaline series. In the Zr/TiO 2 -Nb/Y diagram (Fig. 4-3a), samples plot within sub-alkaline basalt and andesite-basalt fields. Using an FeO t -FeO t /MgO diagram (Fig. 4-3b) to further refine classification, all samples plot as calc-alkaline series. Therefore, the protoliths of the plagioclase-amphibolites are thought to be sodium-rich calc-alkaline basic intrusions. Fig. 4-3 Classification of bafic intrusions in XilinGol Complex. (a) Zr/TiO 2 -Nb/Ydiagram of Winchester (1976); (b) FetO-FetO/MgO diagram of Miyashiro (1974). The primitive mantle-normalized multi-element distribution pattern (Fig. 4-4a) indicates the plagioclase-amphibolite samples are enriched in Pb, U, Th, with negative Nb, Ta and Ti anomalies. The ratios of incompatible elements Nb/Ta= (average is 11.30) and Zr/Hf= (only one sample is a little high at 38.15) are lower than primitive mantle (Nb/Ta=17, Zr/Hf=36, from Sun and Mcdonough, 1989). The total abundances of REE ( ppm, with an average of ppm) are 65

8 Genesis and tectonic significance of basic intrusion in the Xilin Gol Complex low. Chondrite-normalized REE distribution patterns of all the samples are flat without obvious fractionation (Fig. 4-4b): La N /Yb N = with an average of 1.13, while LREE/HREE= with average of There is no fractionation in HREEs: Gd N /Yb N = with an average of The samples show slightly negative Eu anomalies with average of 0.93 (Eu/Eu*= , only one sample is 1.07). Ce shows minor positive anomalies (Ce/Ce*= ). Fig. 4-4 (a) Primitive mantle-normalized multi-element distribution pattern and (b) chondrite-normalized REE distribution pattern for plagioclase-amphitolite. Normalizing values are after Sun and McDonough (1989). Table 4-3 Major and trace elements of plagioclase-amphibolites in Xilin Gol Complex Sample no Major elements (wt %) SiO TiO Al 2 O Fe 2 O FeO MnO MgO CaO Na 2 O K 2 O P 2 O LOI

9 Chapter 4 Total Na 2 O+K 2 O Na 2 O/K 2 O Mg # σ Trace elements (ppm) Be Sc V Cr Co Ni Cu Zn Ga Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm

10 Genesis and tectonic significance of basic intrusion in the Xilin Gol Complex Yb Lu Hf Ta Tl Pb Th U Ti K TREE Parameters La N /Sm N La N /Yb N Ce N /Yb N Gd N /Yb N Sm/Nd Eu/Eu* Ce/Ce* K/K* Nb/Nb* Ti/Ti* Geochronology LA-ICPMS zircon U-Pb dating Most zircons from plagioclase-amphibolite are euhedral and long columnar with high CL intensity and display typical closely-spaced concentric oscillatory zoning, reflecting a magmatic origin. Some grains show a homogeneous texture with high CL intensity and are presumably of metamorphic origin (Fig. 4-5a). Fifteen analyses were carried out on zircons from plagioclase-amphibolite In 11 analyses of magmatic zircon, Th/U ratios cluster tightly from 0.61 to 0.92 (Table to 11), consistent with a magmatic origin (Wu and Zheng, 2004). Thorium contents vary from 66 to 430 ppm and U contents vary from 104 to 706 ppm. The Th/U 68

11 Chapter 4 ratios of the other four analyses are outside the cluster (Table to 15), with 1.37, 0.58, and two instances of Their Th contents vary from 20 to 393 ppm and U contents vary from 35 to 978 ppm. The behaviour of U and Pb is visually represented in Tera-Wasserburg diagram (Fig. 4-5b), where 207 Pb/ 206 Pb is plotted against 238 U/ 206 Pb (Tera and Wasserburg 1972). The plot reveals an array of 11 points above the concordia defining an apparent common Pb discordia line that yields a common Pb-anchored regression intercept age of 309 ± 12 Ma (MSWD=1.8). This age is in good agreement with the weighted mean 206 Pb/ 238 U age of 319 ± 4 Ma (MSWD=1.7), therefore, the 206 Pb/ 238 U ages are considered reliable. A comparably young age of 283 ± 4 Ma was yielded by a spot with quite low Pb, Th and U abundances, implying the removal of such elements during alteration, this age is probably unreliable. The other 3 spots yield 206 Pb/ 238 U ages of 260 ± 3 Ma, 231 ± 3 Ma and 231 ± 3 Ma respectively, which may be related to the amphibolite-facies metamorphism of the sample. Fig. 4-5 Cathodoluminescence images (a) and Tera-Wasserburg concordia diagram (b) of zircons from plagioclase-amphibolite. The white line in (a) is scale with 100μm long, and the circle represents the position of laser-ablation with 32μm diameter. Hollow ellipses in (b) were excluded for mean age calculation either as they have suffered Pb loss or are interpreted to be metamorphic origin Biotite/Hornblende 40 Ar/ 39 Ar dating The 40 Ar/ 39 Ar dating results for biotite from the biotite-plagioclase gneiss and hornblende from plagioclase amphibolite are shown in Table 4-5. Fourteen steps of laser stepwise heating were performed on biotite (Fig. 4-6a). In the first 69

12 Genesis and tectonic significance of basic intrusion in the Xilin Gol Complex three steps, the apparent ages are low and yield a gently rising age spectrum. The gas released during these steps may be from the edges of minerals and the sample may be affected by later geological processes that induced the partial loss of 40 Ar*. Eleven subsequent steps yield a weighted mean age of ± 1.5 Ma with 73.71% 39 Ar release. The total fusion age is ± 1.0 Ma. The K/Ca ratio indicates the biotite separate is not contaminated by other minerals such as plagioclase. As biotite is rich in potassium, the isotope ratios from the sample show very little variation when plotted in an inverse isochron diagram (Fig. 4-6b), making it difficult to calculate a regression line, an isochron age and a reasonable initial 40 Ar/ 36 Ar ratio. Fig. 4-6 Apparent age spectrum and inverse isochron line of biotite from biotite-plagioclase gneiss. 70

13 Table 4-4 LA-ICP-MS zircon U-Pb analytic data for plagioclase-amphibolite in Xilin Gol Complex Concentration/ppm Isotopic Ratio Age/Ma Point No. Th/U Pb Th U 7/6 1s 7/5 1s 6/8 1s 8/2 1s 7/6 1s 7/5 1s 6/8 1s 8/2 1s Abbreviations: 7/6 = 207 Pb/ 206 Pb; 7/5 = 207 Pb/ 235 U; 6/8 = 206 Pb/ 238 U; 8/2 = 208 Pb/ 232 Th.

14 Genesis and tectonic significance of basic intrusion in the Xilin Gol Complex Hornblende was analyzed during thirteen laser heating steps (Fig. 4-7a). For the first four low temperature steps of sample 803-8, the apparent ages are high (> 270Ma), which may be caused by inhomogeneity of the sample or absorption of atmospheric argon into crystal rims. The subsequent eight steps yield a very flat age spectrum with plateau age of ± 1.4 Ma for 83.22% released 39 Ar. The inverse isochron age is ± 6.6 Ma (MSWD=2.51) (Fig. 4-7b). The initial 40 Ar/ 36 Ar ratio is ± 59.1, which is close to the atmospheric ratio of ± 5, indicating no excess Ar was incorporated during hornblende growth. The total fusion age is ± 1.1 Ma. The plateau age is consistent with the isochron age, indicating the credibility of the hornblende 40 Ar/ 39 Ar age. In the last step, the apparent age goes down abruptly, and shows that the Ar has been released radically from the sample. Fig. 4-7 Apparent age spectrum and inverse isochron line of hornblende from plagioclase-amphibolite. 72

15 Chapter 4 Table Ar/ 39 Ar dating results of biotite from biotite-plagioclase gneiss and hornblende from plagioclase-amphibolite Laser energy/w 36 Ar(a) 37 Ar(Ca) 38 Ar(Cl) 39 Ar(K) 40 Ar(r) Age /Ma ±2δ 40 Ar(r) /% 39 Ar(K) /% K/Ca ±2δ / , Biotite, plateau age = ± 1.5 Ma, J-value = ± , total fusion age = ± 1.0 Ma , Hornblende, plateau age = ± 1.4 Ma, J-value = ± , total fusion age = ± 1.1 Ma 73

16 Genesis and tectonic significance of basic intrusion in the Xilin Gol Complex 4.5 Discussion Origin of the basic intrusion The low silica contents (SiO 2 = %), relatively high concentrations of MgO and FeO T ( % and %, respectively) and high Cr contents ( ppm) in plagioclase-amphibolites suggest that their protolith was derived from a mantle source. On the other hand, the moderate Mg # number ( ) and low Ni concentrations ( ppm) indicate that they do not represent primary magmas, and that they may have experienced some crystal fractionation in magma chambers or en route to the surface. The crystal fractionation is also indicated by the lower ratios of incompatible elements Nb/Ta ( ) and Zr/Hf ( ) than the primitive mantle (17 and 36, respectively, from Sun and Mcdonough, 1989). The relatively flat REE patterns (La N /Yb N = ) suggest no garnet crystal fractionation in primary magma. Most samples show slightly negative Eu anomalies (Eu/Eu*= ), suggesting fractionation of clinopyroxene. The positive Eu anomaly of one sample (Eu/Eu*=1.07), may have been caused by plagioclase cumulate in the magma. Fig. 4-8 Tectonic background discrimination diagrams for the protolith of plagioclase-amphibolites on (a) Nb*2-Zr/4-Y diagram (Wood, 1980) and (b) Hf/3-Th-Ta diagram (Meschede, 1986). MORB: Mid-Ocean-Ridge Basalt; VAB: Volcanic Arc Basalt; WPA: Within-Plate Alkali; WPB: Within-Plate Basalt; WPT: Within-Plate Tholeiitic. The samples show low HFS/LREE ratios (Nb/La= ), enrichment in incompatible trace elements (such as Th, U, Pb), and Nb, Ta, Ti negative anomalies, 74

17 Chapter 4 indicating subduction-related mantle metasomatism (Woodhead et al., 2001). As widely documented, post-collisional magmatism commonly show evidence for subduction-related geochemical signatures (e.g. Wang et al., 2004). In ternary diagrams of Nb*2-Zr/4-Y (Fig. 4-8a) and Hf/3-Th-Ta (Fig. 4-8b), all the samples lie in the volcanic arc basalt area, implying the basic intrusion may have formed in a volcanic arc background. However, considering the extensional mechanism during Ma when the basic magma intruded (see next section), the volcanic arc background was probably fed from slab-derived fluids contributing to the magma source region. The related subducting slab should have formed during the Early Paleozoic oceanic subduction along Sonidzuoqi-Xilinhot north-dipping subduction zone Tectonic evolution Along Sonidzuoqi-Xilinhot subduction zone during Early Paleozoic orogeny, north-dipping oceanic subduction induced the overriding continental crust, including the Xilin Gol Complex, to be uplifted, thickened, and then partially molten at 452 ± 5 Ma (Li et al., In press). Continued subduction of the Paleo-Asian Ocean eventually led to collision between the continental accretion belt of the North China Craton and the South Mongolia microcontinent. SHRIMP zircon U-Pb ages of 423 ± 8 and 424 ± 10 Ma from syn-collisional K-rich granites in Sonidzuoqi mark the onset of arc-continent collision in the Sonidzuoqi-Xilinhot area (Shi et al., 2005a). A 40 Ar/ 39 Ar age of 383 ± 13 Ma of Na-amphibole from blueschists associated with ophiolite in the south of Sonidzuoqi (Xu et al., 2001) probably represents cooling after accretionary metamorphism. Most zircons from the plagioclase-amphibolite yield a weighted mean 206 Pb/ 238 U age of 319 ± 4 Ma, which represents the intrusion time of its protolith. This age is consistent with the SHRIMP zircon U-Pb age of 316 ± 3 Ma from garnet-bearing granite in Xilinhot (Shi et al., 2003), 313 ± ± 4 Ma from quartz-diorite in Xilinhot (Bao et al., 2007), 309 ± 8 Ma from quartz-diorite in Sonidzuoqi (Chen et al., 2001) and Ma from bimodal volcanic rocks in Sonidyouqi that next to Sonidzuoqi (Tang et al., 2010). Tang et al. (2010) suggested that the bimodal volcanic rocks formed in a post-collisional extensional setting, thus the simultaneous acid, intermediate and basic magmatic activity indicates that regional extension is responsible for magmatism in the Sonidzuoqi-Xilinhot area. A biotite 40 Ar/ 39 Ar age ± 1.5 Ma from biotite-plagioclase gneiss in Xilin Gol Complex is only slightly younger than the magma activity, which indicates the biotite in the gneiss also recorded this thermal event. The occurrence of unique bimodal magmatism in Xilinhot area at 280 Ma (Zhang et al., 2008) demonstrates regional 75

18 Genesis and tectonic significance of basic intrusion in the Xilin Gol Complex extension lasted to this time, and this continuous extension between Ma suggests that final amalgamation of arc-related terranes in the Solonker zone occurred by the Late Carboniferous. However, there is a substantial body of evidence proving that the final collision of the Solonker zone was between the Late Permian and earliest Triassic (Xiao et al., 2003; Miao et al., 2008; Li et al., 2008; in review). Our plagioclase-amphibolite zircon U-Pb ages of 260 ± 3 and 231 ± 3 Ma, as well as the ± 1.4 Ma hornblende 40 Ar- 39 Ar age for the same sample are consistent with 254 ± 4 Ma SHRIMP zircon U-Pb and 228 ± 21 Ma Rb-Sr whole rock isochron ages from collisional granites in south Sonidzuoqi (Chen et al., 2001). These ages are interpreted to represent the time of amphibolite-facies metamorphism at the time of final collision between the South Mongolia microcontinent and the North China Craton at Ma. The collision between them extended over several tens of millions of years. Extension during Ma was a short-term relaxation after the initial collision during Early Paleozoic convergence. The 222 ± 4 Ma SHRIMP zircon U-Pb age from Sonidzuoqi A-type granites (Shi et al., 2007) records the post-collisional relaxation after the final collision. As the closure temperature of K-Ar system in biotite (~300 C) is lower than that of hornblende (~510 C) (Dodson and McClelland-Brown, 1985), it is striking that the ± 1.5 Ma biotite 40 Ar/ 39 Ar age from the gneiss is older than the ± 1.4 Ma hornblende 40 Ar/ 39 Ar age from plagioclase-amphibolite. This apparent conflict suggests that the amphibolite-facies metamorphism during the final collision did not affect the Xilin Gol Complex. Considering the uplift of the Xilin Gol Complex during the Early Paleozoic orogenesis, the basic intrusion probably intruded into the base of the complex at 319 ± 4 Ma. The extension from Ma induced thinning of the overriding continental crust, including the Xilin Gol Complex. During final collision at Ma, the basic intrusion underwent amphibolite-facies metamorphism and partial melting. The molten mass flowed along pre-existing weak zones to form the plagioclase-amphibolite veins or lenses in the Xilin Gol Complex. The basic intrusion intruded into the base of the Xilin Gol Complex at 319 ± 4 Ma, indicating that extension did not lead to oceanization in the Xilinhot area itself. However, Middle Permian radiolarian fossils from Zhesi formation (P 2 z) were found in Yuejin (Shang, 2004), just ~10 km south of the Xilin Gol Complex. Jian et al. (2010) dated zircons from an N-MORB-like diabase (274.4 ± 2.5 Ma), an E-MORB-like diabase (252.5 ± 2.3 Ma), a transitional sanukitoid/adakite (andesite, ± 2.4 Ma), a sanukitoid (high-mg diorite; ± 1.1 Ma) and an anorthosite (252.2 ± 1.7 Ma) along the Solonker suture. The N-MORB-like diabase contains ca Ma zircon xenocrysts was interpreted to be assimilation of trench sediments when a spreading ridge 76

19 Chapter 4 intersected a trench (Jian et al., 2010). Their new formation ages constrain a magmatic episode in response to slab break-off beneath a fossil forearc, and the youngest xenocryst ages (ca Ma) may define the maximum depositional age of trench sediments. A gabbroic diorite of the Baolidao arc was dated at 310 ± 5 Ma (by SHRIMP) and the Halatu granite was dated at 234 ± 7 Ma (by SHRIMP) (Chen et al., 2008). The combined data suggest the existence of oceanic basins in Middle Permian. Based on zircon U-Pb ages, Hf isotopic ratios and whole-rock Nd-Sr isotope compositions of subduction-related magmas and forearc sediments, Chen et al. (2008) reported zircon age data thus constrain timing of collision between the South Mongolia microcontinent and the North China Craton to have been between 296 and 234 Ma. The Xilin Gol Complex was part of a continent at this time and probably located at the northern margin of the deep basins. The closure of these oceanic basins induced the Late Paleozoic-Early Triassic north-dipping subduction beneath the Xilin Gol Complex and induced the amphibolite-facies metamorphism at the base of the complex. This process suggests a protracted and multilayer evolution of Sonidzuoqi-Xilinhot north-dipping subduction zone, leading up to final collision and suturing in the Central Asian Orogenic Belt Accretionary orogenesis along the Paleo-Asian Ocean Accretionary orogenesis is an important geodynamic process responsible for continental growth. Accretionary orogens can be found of all ages and they have close relationships with supercontinent cycles and plumes. Accretionary orogeny can also be huge both in dimension, reaching several thousands of kilometers in length and width, and time, ranging from 50 million years to several hundreds of millions of years, and usually builds a vast plateau-like orogen with many ophiolitic mélanges and other structures, thus giving rise to a complicated tectonic style (see Xiao et al., 2010, for review). The CAOB is a Precambrian through Early Mesozoic orogenic belt that provides excellent information on accretionary processes and continental growth (Sengör et al., 1993; Jahn et al., 2000; Jahn et al., 2004b; Xiao et al., 2009). A systematic study of its tectonic style of accretionary orogenesis provides constraints for a better understanding of continental growth. However, the tectonic style of the CAOB has been controversial, thus leading to several competing models. For example, Sengör et al. (1993) proposed that the belt formed by progressive accretion in a single, long-lived subduction system until closure of the Paleo-Asian Ocean. In contrast, Mossakovsky et al. (1993) viewed the 77

20 Genesis and tectonic significance of basic intrusion in the Xilin Gol Complex CAOB as a mosaic of exotic and mostly unrelated arc terranes and microcontinents. The Early Paleozoic north-dipping oceanic subduction occurred at 452 ± 5 Ma at the continental marginal arc along Sonidzuoqi-Xilinhot (Li et al., In press). The subducting slab induced asthenospheric convection and slab-derived fluids admixed into the asthenosphere. Therefore the Late Carboniferous (319 ± 4 Ma) basic intrusions in the Xilin Gol Complex show subduction-related geochemical signatures and a volcanic arc background. Many Late Carboniferous arc volcanic and plutonic rocks have been reported in the western section of the CAOB (Wang et al., 2007a; Long et al., 2008; Tang et al., 2010), implying a widespread extension of the CAOB after the Paleozoic accretionary orogenesis and probably marking post-collisional extension (Tang et al., 2010). The southward accretion of the South Mongolia microcontinent resulted in the southward recession of the Paleo-Asian Ocean. However, there were still oceanic basins in Middle Permian along the southern margin of the Xilin Gol Complex. After Late Carboniferous-Early Permian extension at the southern margin of the South Mongolia microcontinent the Late Paleozoic-Early Triassic north-dipping subduction zone became active and brought about the final amalgamation of the CAOB during Late Permian to Middle Triassic by further southward accretion of the continental margin and, finally, collision with the accretionary belt at the northern margin of the North China Craton. Li et al. (In press) suggested that the Xilin Gol Complex recorded long-lived progressive accretion from ~800 Ma to 452 Ma along the southern margin of the South Mongolia microcontinent. The basic intrusions in the complex recorded the post-collisional extension ( Ma) after the Early Paleozoic accretionary orogenesis and the subsequent Late Paleozoic-Early Triassic subduction-accretion until the final amalgamation of the CAOB ( Ma). Therefore, the CAOB formed by progressive accretion of a single, long-lived subduction system that leading the closure of the Paleo-Asian Ocean. 4.6 Conclusion (1) The protolith of the plagioclase-amphibolite exposed in the Xilinhot area is a basic magma, which intruded into the base of the Xilin Gol Complex. The magma of the basic intrusion experienced crystal fractionation and does not represent primary magmas. Slab-derived fluids, which formed during Early Paleozoic oceanic subduction along the Sonidzuoqi-Xilinhot north-dipping subduction zone, admixed into the magma source and produced subduction-related geochemical signatures and volcanic arc background in the 78

21 Chapter 4 basic intrusions of the Xilin Gol Complex. (2) The basic magma intruded at 319 ± 4 Ma as found by LA-ICPMS zircon U-Pb dating. A ± 1.5 Ma biotite 40 Ar/ 39 Ar age from biotite-plagioclase gneiss in the Xilin Gol Complex also recorded this thermal event. Amphibolite-facies metamorphism occurred at ± 1.4 Ma, as indicated by the hornblende 40 Ar/ 39 Ar age, with metamorphic pressure of GPa and temperature of C. (3) There was transient extension during Ma in the Sonidzuoqi-Xilinhot area after the Early Paleozoic oceanic subduction. Extension induced widespread magmatic activity and thinning of the overriding continental crust but did not lead to oceanization in the Xilinhot area. Extension affected almost the entire CAOB and induced widespread Late Carboniferous arc volcanic and plutonic rocks. (4) There were deep marine basins at the southern margin of the Xilin Gol Complex during the Permian. The final collision of Solonker suture zone occurred from 265 to 228 Ma. The closure of the oceanic basins led to the Late Paleozoic- Early Triassic north-dipping subduction beneath the Xilin Gol Complex and induced the amphibolite-facies metamorphism at the base of the complex, but did not affect shallower levels. The Central Asian Orogenic Belt shows a protracted and multilayer collision evolution until suturing occurred. (5) The Xilin Gol Complex, together with the basic intrusions in it, recorded the progressive accretion of a single, long-lived subduction system at the southern margin of the South Mongolia microcontinent from Late Proterozoic (~800 Ma) to Middle Triassic (~228 Ma). 79