Geological record of flat-slab induced extension in the Southern Peruvian forearc

Transcription

1 GSA Data Repository Geological record of flatslab induced extension in the Southern Peruvian forearc Mélanie Noury, Mélody Philippon, Matthias Bernet, JeanLouis Paquette, and Thierry Sempere Contents of this file : Table DR1: Results for zircon UPb ages (insitu and detrital) Table DR2: Results for zircon and apatite fissiontrack ages (insitu and detrital) Text DR1: Methods

2 Lithology Arequipa range (source) 1M22 1M25 3M1 aplite granodiorite ignimbrite Huanca basin (sink) 1M28 1M29 1M30 1M31 red, mid to finegrained sandstone conglomerate matrix red coarse sandstone with conglomeratic layers conglomerate matrix stratigraphic position dyke in Liassic mafic plutonic unit top of Linga laccolith ~30 m above base of the "tuffaceous Fm" capping Huanca sediments ~0.1 km above base of Huanca Fm ~0.9 km above base of Huanca Fm ~1.2 km above base of Huanca Fm ~2.0 km above base of Huanca Fm Lat. ( ) Long. ( ) N CA MA P(χ^2) D (%) Age range P1 P2 P3 P ± ± ± ± ± ± ±120 1 ±2 4 ±2 114 ±7 201 ±16 MSWD: 0.28 MSWD: 0.18 MSWD: 1.6 << << << << ± 1 (41%) 9 ± 1 (35%) 164 ± 5 (7%) 250 ± 8 (6%) 131 ± 2 (43%) 599 ± (21%) 607 ± 7 (33%) 596 ± 8 (30%) 1194 ± 40 (7%) 1205 ± 17 (24%) 12 ± 12 (42%) 1147 ± 12 (41%) 1956 ± 46 (9%) 2247 ± 32 (20%) 23 ± 32 (18%) 2201 ± 28 (23%) Table DR1: Zircon UPb ages. N, number of grains analyzed; P1, P2, P3 and P4 are the population peaks, CA the central age and MA the minimum age calculated using RadialPlotter (Vermeesch, 2012); P(χ^2), probability that the single grain ages represent one population; D, individual grain age dispersion. All ages are reported associated with 2σ standards errors. The bracketed percentages in the P1 to P4 columns indicate the proportion of the total population belonging to this peak.

3 Table DR2 Lithology Crystallization age Lat. ( ) Long. ( ) Mineral dated N ρd (cm 2 ) rse(ρd) U (ppm) P(χ 2 ) (%) D (%) PA CA Mean Dpar Arequipa range (source) 1M09 1M14 (MN) 1M15 1M16 1M17 1M18 (EH) 1M21 (EH) quartzdiorite 89.8 ± 0.7 Ma apatite ± ± pegmatite zircon ± 9 53 ± apatite ± 9 37 ± gabbro quartzdiorite pegmatitic dykes ± 1.1 Ma 82.3 ± 0.4 Ma apatite ± 8 47 ± apatite ± ± Proterozoic apatite << 1 58 ± ± metarenite Proterozoic zircon << 1 56 ± 5 56 ± 5 foliated diorite Liasic zircon << 1 66 ± 7 66 ± 7 apatite ± 57 ± 2.1 1M22 aplite ± 1.2 Ma (n) apatite ± ± 13 1M23 dacite 76 Ma (n) apatite ± 43 ± 2.3 1M24 1M25 1M27 diorite granodiorite quartzdiorite 76.7 ± 0.4 Ma 68.7 ± 0.5 Ma (n) ± 0.8 Ma apatite ± 67 ± apatite ± 57 ± apatite << 1 49 ± ± 25

4 Lithology Huanca basin (sink) 1M31 conglomerate matrix stratigraphic position ~2.0 km above base of Huanca Fm Lat. ( ) Long. ( ) Dated mineral zircon N 0 Age range P1 P2 P3 CA MA D (%) 38 ± 15 (16%) apatite ± 26 (84%) 0 ± 16 (0%) 91 ± ± ± ± M30 red coarse sandstone with conglomeratic layers ~1.2 km above base of Huanca Fm zircon ± 18 (25%) 169 ± 36 (75%) 118 ± ± M29 1M28 conglomerate matrix red, mid to finegrained sandstone ~0.9 km above base of Huanca Fm ~0.1 km above base of Huanca Fm apatite apatite zircon apatite 0 57 ± (0% ) ± (0%) 78 ± 9 (94%) 84 ± 8 62 ± ± 66 (6%) 83 ± 6 77 ± ± 7 50 ± ± (0%) 83 ± 6 79 ± Table DR2: Zircon and apatite fissiontrack ages. Zircon fissiontrack ages determinations were performed by M.N. with ζ = 128 ± 9 and E. Hardwick (EH) with ζ = 174 ± 3 for glass dosimeter CN1. All apatite fissiontrack age determinations were performed by M. N. with ζ = 254 ± 12 for glass dosimeter IRMM540. N, number of grains counted; ρd, dosimeter track density (fluence); rse(ρd), relative standard error on fluence; U, mean uranium content in the analyzed grains (ppm); P(χ 2 ), probability that the single grain ages represent one population; D, age dispersion. All samples of the Huanca basin failed the χ 2 test; P1, P2 and P3 are the population peaks (the bracketed percentages indicate the proportion of the total population belonging to this peak), PA: pooled age, CA: central age and MA: minimum age were calculated using RadialPlotter (Vermeesch, 2012). All ages are reported with 2σ standard error. References for crystallization ages: Demouy et al. (2012); (n) this study (see Table DR1).

5 Text DR1: Methods: In this study we present new geothermochronologic data to better constrain the cooling history of the Arequipa Horst and record of its exhumation in the adjacent Huanca basin (source to sink study). We thus focused our sampling on rocks from the Arequipa Horst (source) which crystallization ages were previously known (Demouy et al., 2012) and we sampled the Huanca basin (sink) deposits from base to top (see main text). In order to perform apatite and zircon fissiontrack as well as zircon UPb analyses, several kilograms of rock specimens were crushed, and apatite and zircons were extracted from the 0200 μm size fraction using standard magnetic and heavy liquid separation techniques. Not all samples yielded enough apatites and/or zircons to carry out analyses. As a result, samples from a total of 16 localities (Fig. 2) could be analyzed among which 11 are localized in the Arequipa range. 1 Fieldwork and synthesis of preexisting maps For this study a detailed structural map (Fig. 2A) has been constructed from available geological maps at the 1:0,000 scale published by the INGEMMET, satellite pictures available within Google Earth, SRTM data, and our own field observations. 2 Zircon UPb dating Zircons extracted from two magmatic rocks of the Arequipa range (Table DR1) an ignimbrite localized above the Huanca basin deposits (sample 3M1), and from four samples of the Huanca Basin were dated using the UPb LAICPMS method. Zircon grains were mounted into epoxy resin blocks and polished to obtain flat surfaces. Excepting sample 3M1, all analyses were performed at the Laboratoire Magmas & Volcans (ClermontFerrand, France). Zircons were handpicked and mounted in epoxy resin, polished in order to expose an internal surface on approximately half their thickness. Insitu LAICPMS UPb analyses were performed using a Resonetics/M50E 193 nm excimer ablation system coupled to a

6 Agilent 7500cs ICPMS. Operating conditions were similar to those described in Paquette and Tiepolo (2007). Calibrations were performed before every analytical session using the NIST SRM 612 reference glass. No common Pb correction was applied owing to the large isobaric interference from Hg. The 235 U signal is calculated from 238 U on the basis of the ratio 238 U/ 235 U= Single analyses consisted of 30 seconds of background integration with laser off followed by 1 minute integration with the laser firing and a 30 seconds delay to wash out the previous sample and prepare the next analysis. Sample 3M1 was analysed at the Institute of Mineralogy and Geochemistry, University of Lausanne. 238 UPb and 235 UPb dates were obtained using a 193nm excimer ablation system UP193FX coupled to an Element XR sector field, single collector ICPMS (Thermo Scientific). Operating conditions were similar to those described in Ulianov et al. (2012). Analyses were performed on the 12 grains available using a 35 μm spot size focused on external zircons growths rims (previously characterized by cathodoluminescence imaging on a Hitachi S2500 SEM at ISterre laboratory, University GrenobleAlpes, France) combined with a relatively low onsample energy density of ~3 J/cm2 and a repetition rate of 5 Hz to minimize fractionation. A GJ1 standard zircon (CA ID TIMS 206 Pb/ 238 U age of ± 0.4 Ma) (Boekhout et al., 2012) was used for external standardization. The Plesoviče zircon standard was measured along with sample zircons on a routine basis to control. No common lead correction was applied due to the presence of 204 Hg in the system. 3 Apatite and zircon fissiontrack dating Apatites and zircons were mounted in epoxy resin and Teflon sheets respectively and polished to expose an internal surface for each grain. Apatites were then etched with a 5.5 mol HNO 3 for 20 s at 21 C. Zircons were etched in a NaOHKOH melt between 20 and 5 hours at 228 C. Two mounts were assembled for each Huanca basin sample in order to be etched for different length of time (e.g. Bernet et al., 2004). In the present study, we used the external

7 detector method for fissiontrack analysis (Gleadow, 1981). Apatite and zircon grain mounts were thus covered with uraniumpoor muscovite sheets as external detectors and irradiated at the FRM II reactor at Garching, Germany. On the one side, apatites samples were irradiated with a nominal fluence of 4.5 x 15 neutrons/cm 2 together with Fish Canyon and Durango age standards and with IRMM540R (15 ppm) dosimeter glasses. On the other side, zircons were irradiated with a nominal fluence of 0.5 x 15 n/cm 2 together with Fish Canyon and Buluk age standards and CN1 (39.8 ppm) dosimeter glasses. After irradiation, the muscovite detectors of all mounts were etched for 18 minutes in 48% HF at 21 C to reveal induced tracks. Spontaneous and induced fission tracks were counted at the ISTerre fissiontrack laboratory, Grenoble University, France, at 1250x magnification using a dry objective and an Olympus BX51 microscope and the FTStage 4.04 system (Dumitru, 1993). Only crystals with polished surfaces parallel to the crystallographic caxis were considered. The individual grain ages as well as the pooled and mean ages were calculated as recommended by Galbraith (2005). To compute central ages, minimum ages and decompose major grain age distributions into components, we used the RadialPlotter software (Vermeesch, 2009, 2012). Due to the low uranium content in apatite, not sufficient horizontally confined tracks were observed to carry out a track length distribution analysis. However, D par values were measured on at least four etch pits per grain. References cited: Bernet, M, Brandon, M.T., Garver, J.I., and Molitor, B.R., 2004, Fundamentals of detrital zircon fissiontrack analysis for provenance and exhumation studies with examples from the European Alps: in Bernet, M., Spiegel, C. (eds) Detrital Thermochronology Provenance Analysis, Exhumation, and Landscape Evolution of Mountain Belts: GSA Spec Pub, 378:2536

8 Boekhout, F., Spikings, R., Sempere, T., Chiaradia, M., Ulianov, A., and Schaltegger U., 2012, Mesozoic arc magmatism along the southern Peruvian margin during Gondwana breakup and dispersal: Lithos, , 48 64, doi:.16/j.lithos Demouy, S., Paquette, J.L., de Saint Blanquat, M., Benoit, M., Belousova, E.A., O Reilly, S.Y., García, F., Tejada, L. C., Gallegos, R., and Sempere T., 2012, Spatial and temporal evolution of Liassic to Paleocene arc activity in southern Peru unraveled by zircon U Pb and Hf insitu data on plutonic rocks: Lithos, 155, , doi:.16/j.lithos Dumitru, T.A., 1993, A new computerautomated microscope stage system for fissiontrack analysis: Nucl. Tracks Radiat. Meas., 21(4), , doi:.16/ (93)90198I. Galbraith, R. F., and Laslett, G. M., 1993, Statistical models for mixed fission track ages : Nucl. Tracks Radiat. Meas. v.17, p Gleadow, A.J.W., Fissiontrack dating methods: what are the real alternatives?, Nuclear Tracks 5, 314 Paquette, J.L., and Tiepolo, M., 2007, High resolution (5 μm) UThPb isotopes dating of monazite with excimer laser ablation (ELA)ICPMS: Chemical Geology, v.240, Ulianov, A., Müntener, O., Schaltegger, U., and Bussy, F., 2012, The data treatment dependent variability of UPb zircon ages obtained using monocollector, sector field, laser ablation ICPMS: J. Anal. At. Spectrom., 27(4), Vermeesch, P., 2009, Radialplotter: A java application for fission track, luminescence and other radial plots: Radiat. Meas., 44(4),

9 Vermeesch, P., 2012, On the visualisation of detrital age distributions : Chem. Geol., , , doi:.16/j.chemgeo