Southern Tibetan Oligocene-Miocene adakites: A record of Indian slab tearing
Southern Tibetan Oligocene-Miocene adakites: A record of Indian slab tearing
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DOI：
10.1016/j.lithos.2014.09.029
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发表时间：
2014-12
期刊：
影响因子：
3.5
通讯作者：
Li Zhang;M. Ducea;L. Ding;A. Pullen;P. Kapp;D. Hoffman
中科院分区：
文献类型：
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作者：
Li Zhang;M. Ducea;L. Ding;A. Pullen;P. Kapp;D. Hoffman
本文分析了南切巴尔瓦地区附近拉萨地体东南部的5个深部岩体样品，确定了它们的地球化学特征，从而更好地了解喜马拉雅东部造山带的地球动力学演化。样品的U-Pb锆石年龄在30 ~ 26 Ma之间，为中硅石，具有较高的K2O（1.5 ~ 8.9）、Th/La比值（0.14 ~ 1.10）、较低的MgO（< 2%）和Mg#（< 50）、较高的初始87sr /86Sri（0.7062 ~ 0.7103）和较低的εNd(t)（- 10.9 ~ - 1.0）同位素组成。此外，这些花岗质和花岗闪长岩具有高Sr/Y（66 ~ 306）和La/Yb(25 ~ 312)，具有埃达质岩石的特征。原位锆石εHf(t)同位素组成在+ 8.1 ~−1.9之间。阿达基岩浆活动开始于喜马拉雅东辐合带附近的30 ~ 26 Ma，向西至日喀则附近的18 ~ 9 Ma有系统的年龄递减。我们将冈底斯弧内埃达克质岩浆活动的时空分布，以及区域渐新世-中新世地质，归因于印度板块的渐进式撕裂。根据阿达克质岩浆活动的年龄自东向西递减的规律，我们推测该裂口起源于东喜马拉雅构造合带下方，并向西扩展。
Oligocene–Miocene granitoids exposed within the 1500-km-long Gangdese arc of southern Tibet exhibit adakitic compositions. Five plutonic samples from the southeastern Lhasa terrane near Namche Barwa area were analyzed to determine their geochemical characteristics and to better understand the geodynamic evolution of the eastern Himalayan syntaxis orogen. The samples, yielded U–Pb zircon ages in the range of 30–26 Ma, are intermediate to silicic in composition and have elevated K2O (1.5–8.9), Th/La ratios (0.14–1.10), low MgO (< 2%) and Mg# (< 50), as well as high initial87Sr/86Sri(0.7062–0.7103) and low εNd(t) (− 10.9 to − 1.0) isotopic compositions. In addition, these granitic and granodioritic rocks have high Sr/Y (66–306) and La/Yb (25–312) ratios, a characteristic of adakitic rocks. In-situ zircon εHf(t) isotopic compositions are in the range of + 8.1 to − 1.9. Within the framework of the Tibetan–Himalayan orogenesis we attribute these rocks to represent partial melting and mixing products of two end-member components of the lower Tibetan crust: the roots of the relatively juvenile Gangdese arc crust and newly-underplated high-potassium mafic magmas. Adakitic magmatism initiated at 30–26 Ma near the eastern Himalayan syntaxis and systematically decreases in age to the west to 18–9 Ma near Shigatse. We attribute this temporal–spatial distribution of adakitic magmatism within the Gangdese arc, along with the regional Oligo-Miocene geology, to the progressive tearing of the Indian plate. Based on the decrease in age of adakitic magmatism from east to west we hypothesize that the tear initiated beneath the eastern Himalayan syntaxis and propagated westward.