Genesis and tectonic setting of Late Jurassic-Early Cretaceous granites in Nachatang area, Central Lhasa Terrane: Constraints from geochemistry, chronology and Hf isotopes

Abstract

西藏中拉萨地块东段大规模侏罗纪-白垩纪花岗岩类的成因类型及构造背景尚未得到有效约束，该时期岩浆作用的时空分布、岩石成因以及深部动力学机制等问题亟需新的深入研究。本文对中拉萨地块东段南缘那茶淌地区的花岗岩类进行了系统的岩相学、元素地球化学、年代学和锆石Hf同位素研究。LA-MC-ICPMS锆石U-Pb定年结果显示，那茶淌地区黑云母二长花岗岩成岩年龄为147±1.4Ma，花岗闪长岩年龄为140.6±1.3Ma，系晚侏罗世-早白垩世中酸性岩浆活动的产物。在元素地球化学组成上，黑云母二长花岗岩和花岗闪长岩的主量元素组成具有富SiO₂（分别为71.02%~71.81%和65.17%~66.73%）、Al₂O₃（分别为13.45%~13.57%和14.43%~15.20%）、碱金属（Na₂O+K₂O）（分别为6.79%~7.48%和6.55%~7.37%），贫TiO₂（分别为0.15%~0.21%和0.10%~0.13%）等特征，显示Ⅰ型准铝-弱过铝质（A/CNK=0.98~1.14）的高钾钙碱性系列特征。微量元素以富集K、Rb、Ba、Th等大离子亲石元素，亏损Ta、Nb、P、Ti等高场强元素为特征。花岗闪长岩稀土元素总量（ΣREE=263.9×10^-6~313.8×10^-6）高于黑云母二长花岗岩（ΣREE=116.3×10^-6~168.8×10^-6），均表现为轻稀土元素富集，重稀土元素相对亏损，具弱的负Eu异常。两种岩石锆石Hf同位素均以负的ε_Hf（t）值和古老的地壳模式年龄为特征，黑云母二长花岗岩ε_Hf（t）值为-21.64~-7.66，地壳模式年龄（t_DM^C）为1.68~2.56Ga，花岗闪长岩锆石ε_Hf（t）值为-11.95~-8.15，地壳模式年龄（t_DM^C）为1.71~1.95Ga。结合前人研究资料，认为中拉萨地块东段那茶淌地区晚侏罗世-早白垩世花岗岩类形成的构造背景与班公湖-怒江洋南向俯冲有关，岩浆来源于俯冲作用诱发古老下地壳物质的重熔。;The genetic types and tectonic settings of the large-scale Jurassic-Cretaceous granites in the eastern part of the Central Lhasa Terrane in Tibet have not been well constrained, and the temporal and spatial distribution of their magmatism, petrogenesis and deep dynamic mechanism in this period are in urgent need of further research. In this paper, the lithography, chronology, geochemistry and zircon Hf isotope of the biotite monzogranite and granodiorite from the Nachatang area in the southern margin of the eastern segment of the Central Lhasa Terrane are systematically studied. LA-MC-ICPMS zircon U-Pb dating results show that the biotite monzogranite and granodiorite are 147±1.4Ma and 140.6±1.3Ma, respectively, showing they are the products of magmatism in Late Jurassic-Early Cretaceous. In terms of geochemical compositions, the biotite monzogranite and granodiorite have low TiO₂(0.15%~0.21% and 0.10%~0.13%), high SiO₂ (71.02%~71.81% and 65.17%~66.73%), Al₂O₃ (13.45%~13.57% and 14.43%~15.20%) and alkali (Na₂O+K₂O) (6.79%~7.48% and 6.55%~7.37%) contents, showing the characteristics of Ⅰ-type peraluminum to weak peraluminum calc-alkaline series (A/CNK=0.98~1.14). Trace element compositions of the measured samples are characterized by enrichment of large ion lithophile elements such as K, Rb, Ba and Th, and depletion of high field strength elements such as Ta, Nb, P and Ti. The total amount of REE in the granodiorite (ΣREE=263.9×10^-6~313.8×10^-6) is higher than that in the biotite monzonite (Σ REE=116.3×10^-6~168.8×10^-6), which shows the enrichment of LREE and relative depletion of HREE, with weak negative Eu anomaly. The zircon Hf isotopes are characterized by negative ε_Hf(t) values and ancient crustal model ages. The ε_Hf(t) values of the biotite monzogranite range from -21.64 to -7.66, and the crustal model ages (t_DM^C) range from 1.68Ga to 2.56Ga. Zircon ε_Hf(t) values of the granodiorite vary from -11.95 to -8.15, and the crustal model ages (t_DM^C) from 1.71Ga to 1.95Ga. Finally, we conclude that the tectonic setting of Late Jurassic to Early Cretaceous granites in the Nachatang area is related to the southward subduction of the Bangong-Nujiang Ocean seafloor, and the magma is derived from the remelting of ancient lower crust material induced by subduction.