Timing of E-W Extension Deformation in North Himalaya: Evidences from Ar-Ar Age in the Cuonadong Dome, South Tibet
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摘要: 特提斯喜马拉雅带以广泛发育近E-W向和近S-N向断裂以及北喜马拉雅片麻岩穹隆带为典型特征.藏南错那洞穹隆位于特提斯喜马拉带的东部,是近两年新发现并厘定的穹隆构造.该穹隆从外向内主要由3部分组成:上部单元(盖层)、中部单元(滑脱系)和下部单元(核部),其中滑脱系主要由一套强烈变形的片岩、伟晶岩、花岗岩、大理岩和矽卡岩组成,片岩包括含石榴石云母片岩、含石榴石十字石云母片岩、含蓝晶石石榴石十字石片岩、含矽线石蓝晶石石榴石片岩和云母石英片岩.野外构造变形特征表明滑脱系为一条强烈变形的韧性剪切带,发育大量的鞘褶皱、"Z"形揉褶皱和眼球状构造、石榴石的旋转碎斑、S-C组构和压力影构造.错那洞穹隆记录了4期构造变形:第1期由北向南的逆冲挤压构造、第2期由南向北的韧性伸展构造、第3期近E-W向的韧性伸展构造变形和第4期成穹后的脆性垮塌构造.通过对滑脱系中含石榴石云母片岩的白云母进行Ar-Ar同位素测年,获得坪年龄为14.0±0.2 Ma,等时线年龄为13.7±0.5 Ma,二者基本一致,同时微观构造特征显示石英呈亚颗粒旋转重结晶(SGR),其韧性变形的温度为450~550℃,该变形温度高于白云母的封闭温度.因此,白云母Ar-Ar坪年龄(14.0±0.2 Ma)代表错那洞穹隆近E-W向伸展变形的时间,也即近S-N向桑日-错那裂谷的活动时间.结合构造变形和年代学特征,认为错那洞穹隆是STDS向北伸展拆离的主导机制叠加后期近E-W向韧性伸展活动的结果.Abstract: The Tethys Himalaya is characterized by the S-N trending and E-W trending structures, and the North Himalaya gneiss domes (NHGD).The Cuonadong dome, located at the eastern part of the North Himalaya, is a recently newly identified dome.The Cuonadong dome is divided into three units from outer to inner:the upper unit (the cover rocks), middle unit (the detachment layer) and lower unit (the core).The middle unit mainly consists of a series of strong deformation schist, pegmatite, granite, marble, and skarn.The main schist types include garnet mica schist, garnet staurolite schist, kyanite garnet staurolite schist, sillimanite kyanite garnet schist, and mica quartz schist.This unit is characterized by a ductile shear zone including the sheath fold, augen structure, rotating porphyroclast, S-C structure, and pressure shadow structure.The Cuonadong dome preserves evidences for four major deformational events:N-S thrust (D1), early approximately S-N extensional deformation (D2), approximately E-W extensional deformation (D3), and collapse structural deformation (D4).Ar-Ar dating of muscovite from the mylonitic schist in the Cuonadong dome yielded Ar-Ar plateau age of 14.0±0.2 Ma and inverse isochron age of 13.7±0.5 Ma, meanwhile the presence of subgrain rotation recrystallization (SGR) in quartz shows that the schist was deformed under high deformation temperature (450~550℃), which is clearly higher than the closure temperature of muscovite.Therefore, we suggest that the Ar-Ar plateau age of 14.0±0.2 Ma represents the age of the E-W extensional deformation in the Cuonadong dome, also the age of the S-N trending Sangri-Cuona rift.Combined with the structural deformation and thermochronology, we suggest that the formation of the Cuonadong dome resulted from both the earlier S-N and later E-W extensional deformations, especially the S-N extensional deformation, i.e.the STDS.
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Key words:
- Ar-Ar age /
- extension deformation /
- Cuonadong dome /
- North Himalaya /
- geochemistry /
- geochronology
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图 1 北喜马拉雅构造格架及其矿产分布
图a为喜马拉雅造山带中南部局域构造简图及北喜马拉雅片麻岩穹隆带(NHGD)分布,据Lee et al.(2004)修改;GHS.高喜马拉雅,LHS.低喜马拉雅,MBT.主边界逆冲断裂,MCT.主中央逆冲断裂,STDS.藏南拆离系,GKT.吉隆-康马逆冲断裂.图b为北喜马拉雅东段构造格架及其矿产分布,据Sun et al.(2016)修改
Fig. 1. Tectonic framework and deposits in North Himalaya
图 3 错那洞穹隆岩石-构造单元示意剖面(a)和野外岩性特征(b~g)
a.错那洞穹隆岩石-构造单元示意剖面图及变质矿物分带,据Fu et al.(2018)修改;b.上部单元中含红柱石粉砂质板岩;c.上部单元中含堇青石粉砂质板岩;d.石榴石十字石云母片岩;e.石榴石十字石云母片岩,十字石颗粒明显变大;f.含十字石石榴石云母片岩,可见石榴石的旋转斑晶和顺片岩面理侵入并变形的花岗岩;g.蓝晶石石榴石云母片岩.Bt.黑云母;Grt.石榴石;St.十字石;Sil.矽线石;Ky.蓝晶石
Fig. 3. Schematic section (a) and field lithologic characteristics (b-g) of the rock-tectonic unit through the Cuonadong dome
图 5 错那洞穹隆野外构造变形特征
a.错那洞穹隆北部上部单元粉砂质板岩在挤压作用下呈M或W型褶皱,代表了早期由北向南逆冲构造;b.错那洞穹隆北部上部单元石英脉呈石香肠状产出,指示向北伸展剪切;c.错那洞穹隆东部中部单元中鞘褶皱的野外特征,照片面为YZ面,拉伸线理产状为350∠18°,代表第2期由南向北韧性剪切特征;d.错那洞穹隆北部中部单元的鞘褶皱,强变形的花岗岩呈一系列透镜体产出,其表面可见明显的拉伸线理,线理产状为348∠20°,代表第2期由南向北韧性剪切特征;e.错那洞穹隆东部中部单元含石榴石云母片岩中石榴石的旋转斑晶,指示右行,代表了第3期近E-W向韧性变形特征;f.错那洞穹隆北部中部单元变形的伟晶岩,指示右行,代表了第3期近E-W向韧性变形特征
Fig. 5. Field structural deformation characteristics at different units in the Cuonadong dome
表 1 错那洞穹隆含石榴石云母片岩样品CND01-3中白云母Ar-Ar测年结果
Table 1. Muscovite Ar-Ar dating results of sample CND01-3 in the Cuonadong dome
序号 T(℃) (40Ar/39Ar)m (36Ar/39Ar)m (38Ar/39Ar)m 40Ar(%) F 39Ar(1014) 39Ar(Cum.;%) t(Ma) ±1σ(Ma) 1 650 164.130 9 0.526 1 0.148 0 5.28 8.662 1 0.04 0.07 51 18 2 720 42.658 8 0.139 8 0.031 2 3.12 1.331 3 0.34 0.72 7.9 1.3 3 740 21.811 0 0.066 0 0.000 6 10.52 2.294 6 0.30 1.30 13.5 1.1 4 770 10.842 7 0.034 2 0.007 7 6.83 0.740 1 0.39 2.04 4.4 1.3 5 800 10.056 0 0.034 0 0.019 1 0.02 0.001 6 0.38 2.77 9.7 1.4 6 830 9.857 0 0.025 0 0.018 2 25.04 2.468 6 1.52 5.70 14.56 0.42 7 850 5.556 2 0.010 4 0.014 5 44.86 2.492 4 2.71 10.91 14.70 0.23 8 870 5.150 7 0.009 0 0.014 2 48.38 2.492 1 2.44 15.61 14.70 0.25 9 890 4.339 5 0.006 2 0.013 4 57.95 2.514 7 4.91 25.04 14.83 0.19 10 910 3.119 7 0.002 5 0.013 2 76.53 2.387 6 8.50 41.39 14.09 0.15 11 1 000 2.963 3 0.002 1 0.013 0 78.68 2.331 5 11.08 62.69 13.76 0.14 12 1 070 3.604 2 0.004 6 0.013 6 62.02 2.235 5 6.95 76.05 13.19 0.14 13 1 160 9.216 5 0.025 8 0.016 9 17.34 1.598 2 7.72 90.90 9.44 0.13 14 1 300 11.142 2 0.033 5 0.018 0 11.05 1.231 2 4.73 100.00 7.28 0.13 注:下标m代表样品中测定的同位素比值;(37Aro/39Ar)m= 0.000 0;W=17.88 mg;J=0.003 283;F=40Ar*/39Ar. -
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