藏南冈底斯地体谢通门—曲水韧性剪切带40Ar/39Ar年代学约束
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本文为国家自然科学基金项目(编号:40921001、41430212、41202153、41472198)、中国地质调查局项目(编号:1212010818094、12120115026801、D1515 2)和国土资源部公益性行业基金项目(编号:201511022、201211093)联合资助的成果。


The 40Ar/39Ar Geochronological Constraints on the Xaitongmoin—Quxu Ductile Shear Zone in the Gangdese Batholith, Southern Xizang(Tibet)
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    摘要:

    藏南冈底斯谢通门—曲水韧性剪切带发育在拉萨地体南缘,其活动时限对厘定藏南大地构造演化极为关键,并且对揭示印度—亚洲陆陆碰撞前缘的构造变形具有重要的意义。谢通门—曲水韧性剪切带位于拉萨地体冈底斯岩基南缘,为一条向北倾覆,走向近东西,宽1~2 km的滑覆型韧性剪切带,其中剪切带中的糜棱岩可见明显的面理,并且在面理上发育有南北向拉伸线理,为一套典型的S—L构造岩。为了定量确定剪切带的变形温度,本文在宏观和微观分析的基础上,对糜棱岩中的典型样品进行了EBSD石英组构分析,分析结果表明谢通门—曲水韧性剪切带以底面和柱面滑移为主,伴有少量菱面滑移,变形温度为中低温(300℃~550℃),即绿片岩相到低角闪岩相。通过对剪切带中的绢云母和黑云母40Ar/39Ar测年,获得了较好的年代学数据,两组年龄21.81±0.28 Ma和23.24±0.32 Ma在误差范围内具有一致的年代学意义,结果表明谢通门—曲水韧性剪切带形成于中新世早期(Aquitanian期)。结合云母的40Ar-39Ar年龄以及区域大地构造背景,谢通门—曲水韧性剪切带形成于板块后碰撞阶段(postal collision),是拉萨地体由挤压向伸展转换的产物。该剪切带年代学的厘定,一方面限定了拉萨地体南缘新生代大地构造演化的过程,另一方面对造山带构造以及青藏高原中—南部地区的地质研究具有推进作用。

    Abstract:

    The Gangdese batholith, located in south margin of Lhasa terrane, is mainly composed of granitoid rocks, which were related to the subduction of the Neo Tethys and collision of Indian—Asian plates. The Xaitongmoin—Quxu shear zone, lied in the southern margin of Lhasa terrane, is a detachment shear zone, with a width of 1~2 km, extending from the west to east and dipping to the north. Mylonites from the shear zone demonstrates obvious foliation, and stretching lineation observed in the foliation reveals a typical S—L structural tectonites. Achievements and studies were mainly on geochemistry, geochronology and petrogenesis. However, geological structural research is not well understood, especially about the Xaitongmoin—Quxu shear zone. Based on this issue, we focus on the geochronology of the shear zone to provide new and accurate constraints on formation of the Xaitongmoin—Quxu shear zone.Methods: Biotite and sericite grains from 2 crushed samples were handpicked under a binocular microscope, then selected by a magnetic separator and cleaned in an ultrasonic ethanol bath. The analyzed grains should be over 99% for the purity. The two samples were irradiated for over 24 hours in the nuclear reactor at the Chinese Institute of Atomic Energy in Beijing and then cooled for about 3 months. Mica 40Ar 39Ar step heating analysis was conducted by mean of an MM 1200B mass spectrometer at the key laboratory of isotopic geology of CAGS. The measured isotopic ratios were adjusted or corrected for mass discrimination atmospheric argon, blanks and irradiation induced mass interference. The correction factors of interfering isotopes produced during irradiation were determined by analyzing the irradiated pure K2SO4 and CaF2. The values were listed below: (36Ar/37Ar)Ca=0.000240, (40Ar/39Ar)K=0.004782, and (39Ar/37Ar)Ca=0.000806. The decay constant was λ=5.543×10-10/a. The detailed procedures and calculations are referred by Chen et al.(2006, 2011) and Zhang et al.(2006). Crystallographic preferred orientation (CPO) measurements of quartz were analyzed using the EBSD (electron back scattered diffraction) technique at the key laboratory of continental and dynamics of Institute of Geology, CAGS. The EBSD technique is a scanning electron microscope JEOL JSM 5610LV(acceleration voltage for 20 kv; working distance for 20 mm). Thin sections for the EBSD measurements were cut parallel to the structural XZ plane (X parallel to the lineation, Y parallel to foliation and normal to lineation and Z normal to foliation). Detailed analytical procedures were given by Xu et al. (2009).Results: The study results suggest that basal slip system and prismatic slip system are dominant with little rhombohedral slip, indicating a low—middle temperatures (300~550℃), namely from greenschist facies to low amphibolite facies. In addition, by means of dating of 40Ar 39Ar of sericite and biotite from mylonites of the Xaitongmoin—Quxu ductile shear zone, we obtained good 40Ar 39Ar data (21.81 Ma and 23.74 Ma). Conclusions: The data has the similar geologic significance in essence, and shows that formational age of the shear zone might be early period of the Miocene (21.81~23.74 Ma). Combined with tectonic setting in southern Tibet, it belongs to the post collision of Indian—Asian plates, as having a close relationship with tectonic transfer from compression to extension in southern Tibet. Moreover, identification of the Xaitongmoin—Quxu ductile shear zone provides accurate timing constraints on Cenozoic tectonic evolution process, and further advances geologic studies of the middle—south Tibetan plateau.

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孟元库,许志琴,马士委,杨斐斐.2016.藏南冈底斯地体谢通门—曲水韧性剪切带40Ar/39Ar年代学约束[J].地质论评,62(4):795-806,[DOI].
MENG Yuanku, XU Zhiqin, MA Shiwei, YANG Feifei.2016. The 40Ar/39Ar Geochronological Constraints on the Xaitongmoin—Quxu Ductile Shear Zone in the Gangdese Batholith, Southern Xizang(Tibet)[J]. Geological Review,62(4):795-806.

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  • 收稿日期:2015-12-31
  • 最后修改日期:2016-04-16
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  • 在线发布日期: 2016-07-22
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