拉萨地体西段达若地区古新世花岗斑岩成因:锆石U-Pb年代学、岩石地球化学和Sr-Nd-Pb-Hf同位素的约束

前人对林子宗群典中组火山岩的成因研究较为深入，却忽略了侵位于其中的大量花岗斑岩.在野外地质调查的基础上，对拉萨地体西段达若地区花岗斑岩进行了年代学、岩石地球化学和Sr-Nd-Pb-Hf同位素研究.结果显示，2件花岗斑岩的成岩年龄分别为61.9±0.3 Ma(MSWD=0.17)和61.1±0.6 Ma(MSWD=0.69)，为古新世岩浆活动的产物；岩石中未见角闪石及富铝矿物，属高钾钙碱性－钾玄岩系列，具有高SiO₂(76.16%~82.78%，平均为78.28%)、高碱(K₂O+Na₂O=4.16%~6.93%，平均为6.09%)、低CaO(0.11%~0.16%，平均为0.14%)和P₂O₅(0.02%~0.04%，平均为0.03%)的特点，富集Rb、Th、K和LREE，亏损Ba、Nb、Sr、P、Ti和HREE，轻、重稀土元素分馏强烈，负Eu异常显著，属强过铝质的高分异I型花岗岩.岩石富含放射成因Pb，(208Pb/204Pb)_t、(207Pb/204Pb)_t和(206Pb/204Pb)_t值分别为为38.737~38.944、15.661~15.682和18.079~18.624，且具有较高的(87Sr/86Sr)_i值(0.722 739~0.744 497)、ε_Nd(t)值(－6.82~－6.67)，锆石ε_Hf(t)值(－4.97~－1.54)为较为分散而低弱的负值，Hf同位素二阶段亏损地幔模式年龄(T_DM2)介于1 083~1 273 Ma，Nd-Hf同位素之间发生了一定程度的解耦.综合研究表明，达若花岗斑岩形成于印度－欧亚大陆主碰撞板块汇聚(65~41 Ma)的早阶段，主要为滞后的俯冲新特提斯洋壳与地幔岩石相互作用形成的母岩浆底侵于拉萨地体古老地壳之下致使其重熔，并与少部分幔源岩浆混合之后，经高程度的结晶分异作用形成. 

Petrogenesis of Paleocene Granite Porphyry of Daruo Area in Western Lhasa Block,Tibet: Constraints from Geochemistry,Zircon U-Pb Chronology and Sr-Nd-Pb-Hf Isotopes

The genesis of volcanic rocks in Dianzhong Formation of Linzizong Group has been deeply studied, but a large number of granitic porphyry emplaced has been neglected. Based on the field geological survey, the chronology, petrogeochemistry and Sr-Nd-Pb-Hf isotopes of granitic porphyry of Daruo area in the western Lhasa block were studied. The results show that the diagenetic ages of the two granitic porphyry samples are 61.9±0.3 Ma (MSWD=0.17) and 61.1±0.6 Ma (MSWD=0.69), respectively, indicating that they are products of Paleocene magmatic activities. Hornblende and aluminum-rich minerals are not found in the rocks, indicating they belong to the series of high potassium calc-potassium basaltic rocks, which are characterized by high SiO₂ (76.16%-82.78%, average 78.28%), high alkali (K₂O+Na₂O=4.16%-6.93%, average 6.09%), low CaO(0.11%-0.16%, average 0.14%) and P₂O₅(0.02%-0.04%, average 0.03%). It is enriched in Rb, Th, K and LREE, and depleted in Ba, Nb, Sr, P, Ti and HREE. LREE and HREE have strong fractionation, and the negative Eu is very significant. These characteristics indicate that it belongs to weakly peraluminous and highly fractionated I-type granites. Daruo granite porphyry is rich in radiogenetic Pb, and the values of (208Pb/204Pb)_t, (207Pb/204Pb)_t and (206Pb/204Pb)_t are 38.737-38.944, 15.661-15.682 and 18.079-18.624, respectively. Meanwhile, the granite porphyry has high (87Sr/86Sr)_i value (0.722 739-0.744 497) and ε_Nd(t) values (-6.82—-6.67), the ε_Hf(t) values of zircon are only weakly and diffusely negative, and T_DM2 is between 1 083 and 1 273 Ma, which means that there is a certain degree of decoupling between Nd-Hf isotopes. According to the comprehensive study, Daruo granite porphyry was formed in the early stage of the main collision convergence of the India-Eurasia plate (65-41 Ma), and the parent magma, formed by the interaction of the hysteretic subduction Neo-Tethys oceanic crust and mantle rocks, underplated beneath the ancient crust of Lhasa block, causing it to remelt and mix with a small part of mantle magma, then the granite porphyry was generated after a high degree of crystallization differentiation.