Oligocene-Miocene high Sr/Y magmatism and implications for deep processes of Qulin pluton in Gangdese batholith, southern Tibet

Abstract

曲林岩体位于冈底斯带中段的南缘，为渐新世-中新世复合岩体，主体为粗粒花岗斑岩，被后期煌斑质、花岗闪长质和花岗质岩脉切割，是多期岩浆作用的产物，出露面积约8km²。花岗斑岩两组样品（T0849-PG和T0849-G）的锆石U-Pb定年结果分别为29.7±0.1Ma和30.0±0.2Ma。花岗斑岩为高钾，准铝质，低MgO，高度富集轻稀土元素（LREE）和大离子亲石元素（LILE），亏损重稀土元素（HREE）和高场强元素（HFSE）。此外具有高Sr、Sr/Y、（La/Yb）_N；低Y和Yb，弱Eu负异常等特征。岩体内发育一系列近南北向展布的花岗闪长玢岩脉，其中两组样品的锆石U-Pb定年结果分别为15.5±0.1Ma（T0848-PY）和14.4±0.1Ma（T0850）。两条花岗闪长玢岩脉具有与岩体主体相似的稀土和微量元素分布模式，同样富集轻稀土及大离子亲石元素，亏损重稀土及高场强元素。花岗斑岩的⁸⁷Sr/⁸⁶Sr_（i）=0.706102~0.706202，ε_Nd（t）=-0.6~+0.6，锆石ε_Hf（t）=+4.9~+7.9；花岗闪长玢岩脉的⁸⁷Sr/⁸⁶Sr_（i）=0.705429~0.705474，ε_Nd（t）=-1.4~-0.2，锆石ε_Hf（t）=+2.6~+7.6。本文数据和文献数据结果表明：曲林花岗斑岩与花岗闪长玢岩脉均来源于加厚南拉萨下地壳的部分熔融，很可能与增厚的深部岩石圈的拆沉或俯冲印度岩石圈的撕裂诱发软流圈上涌有关。;The Qulin pluton is located at the southern edge of the middle part of Gangdese belt. As an Oligocene-Miocene pluton, its main body is coarse-grained granite porphyry, which was later staged by lamproitic, granodioritic, and granitic dikes which are the products of multi-stage magmatism, with an exposed area of about 8km². The zircon U-Pb dating results of two samples of granite porphyry are ~30Ma. Whole rock geochemical studies show that granite porphyry is high potassium and quasi-aluminum, characterized by low MgO together with enriched light rare earth elements (LREE) and large ion lithophile elements (LILE), depleted heavy rare earth elements (HREE) and high field strength elements (HFSE). In addition, it has the characteristics of high Sr, Sr/Y, (La/Yb)_N; low Y and Yb, and weak Eu negative anomaly. A series of nearly N-S trending granodiorite porphyrite dikes occur in the pluton, among which the zircon U-Pb dating results of two samples are 15.5±0.1Ma and 14.4±0.1Ma, respectively. They have similar distribution patterns of rare earth and trace elements as the main body of pluton. And they are also rich in LREE and LILE, along with depleting HREE and HFSE. The ⁸⁷Sr/⁸⁶Sr_(i) values of granite porphyry range from 0.706102 to 0.706202, ε_Nd(t) of which from -0.6 to +0.6, and zircon ε_Hf(t) of which from +4.9 to +7.9; while for granodiorite porphyrite dikes, the ⁸⁷Sr/⁸⁶Sr_(i) values range from 0.705429 to 0.705474, ε_Nd(t) from -1.4 to -0.2, and zircon ε_Hf(t) from +2.6 to +7.6. The new data and literature data results show that: both the granite porphyry and granodiorite porphyrite dikes of the Qulin pluton origined from the partial melting of the thickened lower crust beneath Lhasa terrane, possibly related to the upwelling of asthenosphere induced by delamination of thickened lithosphere or tearing of the Indian lithospheric slab.