青海共和盆地周缘印支期花岗岩类的成因及其构造意义

Granitoids around Gonghe basin in Qinghai province:petrogenesis and tectonic implications

本文对青海共和盆地周缘印支期黑马河岩体、温泉岩体、大河坝岩体和同仁岩体花岗闪长岩进行了主量元素、微量元素和 Pb-Sr-Nd 同位素地球化学研究,并对黑马河岩体和温泉岩体进行皓石 U-Pb LA-ICP-MS 年代学研究。结果表明, 黑马河岩体的岩浆结晶年龄为235±2Ma,属印支早期,而温泉岩体的岩浆结晶年龄为218±2Ma,属印支晚期。这些印支期花岗闪长岩的 SiO_2=63.34～68.06%,K_2O/Na_2O=0.82～1.36,岩石均为准铝质(A/CNK=0.9～1.0),并属中钾到高钾钙碱性岩系。它们总体上具有相似的微量元素组成特征,并有着极为相似的稀土元素组成模式,(La/Yb)_N 值主要介于10～15之间,存在微弱到中等程度的负 Eu 异常(Eu/Eu~*=0.5～0.8)。岩石初始 Sr 同位素比值 I_(Sr)=0.70701-0.70952,ε_(Nd)(t)=-3.8到-8.4,指示它们的岩浆物质主要来自于地壳物质的部分熔融。这些岩石以高放射成因 Pb 同位素组成为特征,其全岩初始 Pb同位素比值为:(^(206)Pb/^(204)Pb)_t=18.068～18.748、(^(207)Pb/^(204)Pb)_t=15.591～15.649、(^(208)Pb/^(204)Pb)_t=38.167～38.554。地球化学特征指示共和盆地周缘印支期花岗岩类的原岩为下地壳变玄武岩类,并且这类原岩可能派生于元古宙富集地幔,但在不同区段,下地壳变玄武岩类存在着一定程度化学组成的不均一性。根据花岗岩类对深部地壳物质的地球化学示踪及其区域对比,共和盆地周缘的西秦岭、柴达木(包括东昆仑)和欧龙布鲁克块体具有统一的地壳基底组成,并具有扬子型块体的构造属性。结合区域地质背景的分析,共和盆地周缘印支早期花岗岩类(以黑马河岩体为代表)可能形成于俯冲陆壳断离的地球动力学背景,而印支晚期花岗岩类(以温泉岩体为代表)形成于中央造山带在地壳加厚作用后岩石圈拆沉作用的地球动力学背景。

This paper reports geochemical and Pb-Sr-Nd isotopic compositions from four Indosinian granitoid intrusions （the Heimahe, Wenqian, Daheba and Tongren intrusions ）, together with U-Pb zircon LA-ICPMS ages from two intrusions （the Heimahe and Wenqian intrusions） around the Gonghe Basin in Qinghai Province, The results show that the U-Pb zircon ages for the Heimahe and Wenqian intrusions are 235 2Ma and 218 2Ma, respectively. The granitoids from the four intrusions display SiO2 contents of 63.34 - 68.06% and K2O/Na2O ratios of 0.82 -1.36. They are metaluminous with A/CNK = 0.89 -1.0 and belong to medium-high potassium calc-alkaline series, Generally, they have similar trace element geochemical characteristics and REE patterns with （La/Yb） N = 10- 15 for most samples and Eu/Eu^＊ = 0.5 -0, 8, Whole-rock initial Sr isotopic ratios （Isr） ranging from 0, 70701 to O. 70952, and εNd （t） values from -3.8 to -8.4, indicating that their magma sources were dominantly derived from partial melting of continental crustal materials. They are characterized by high radiogenic Pb isotopic compositions with （^206Pb/^204Pb）t = 18. 068 - 18.748, （^207Pb/^204Pb）t= 15.591 - 15. 649 and （^208Pb/^204Pb）t =38.167 - 38.554. The geochemistry of the granitoids reveals that their magma sources were high-potassium meta-basalts in lower crust, which could be derived from an enriched mantle source in Proterozoic. However, these meta-basalts have small extent chemical heterogeneity, The geochemical probing to deep crustal compositions, combined with regional comparisons, indicates that the West Qinling, the Qaidam （including east Kunlan） and the Oulonghuluke blocks around the Gonghe basin have a common crustal basement, which have an affinity with the Yangtze block. According to analyses of regional geological setting, the early Indosinian （eg. the Heimabe intrusion） and the late Indosinian granitoids （eg. the Wenqian intrusion） have distinct geodynamie processes for their magma generation. The former was suggested to be related to break-off of a subdueted slab; the latter resulted from lithospherie delemination after crustal thickening due to collision between continental plates in Triassic.