川西李家沟锂多金属矿区晚三叠世花岗细晶岩脉的成因：地球化学、锆石U-Pb年龄和Hf同位素的证据

Petrogenesis of granite aplite in the Lijiagou lithium polymetallic ore district in western Sichuan: constraints from geochemistry, zircon U-Pb geochronology and Hf isotope

关键词:  锆石U-Pb-Hf同位素  S型花岗岩  后碰撞  古特提斯  李家沟锂多金属矿床  四川可尔因地区

Key words:  zircon U-Pb-Hf isotopes  S-type granite  post-collisional  paleotethys  Lijiagou Li polymetallic deposit  Keeryin area in Sichuan

摘要:

李家沟锂多金属矿床位于松潘–甘孜褶皱造山带东南缘，是近年来在区内发现的超大型锂多金属矿床，矿区内广泛发育花岗细晶岩脉。本文对花岗细晶岩脉进行了全岩地球化学测试、锆石U-Pb定年和Hf同位素分析，探讨其岩石成因及地质意义。花岗细晶岩锆石U-Pb加权平均年龄为200±2 Ma，属晚三叠世—早侏罗世。岩石具高硅（w（SiO₂）=73.27%~75.14%）、富铝（w（Al₂O₃）=14.8%~15.25%）、富碱（w（Na₂O+K₂O）=4.86%~8.08%）、贫钙（w（CaO）=0.45%~1.45%）的特征，A/CNK值均大于1.1，属中钾–高钾钙碱性强过铝质S型花岗岩。微量元素表现出Rb、Th、U、K的正异常和Ba、Sr、P、Ti相对负异常。稀土元素显示轻稀土富集、重稀土亏损及弱的负Eu异常（δEu=0.68~0.81）。岩石中锆石ε_Hf（t）值和对应的二阶模式年龄（t_DM2）较为均一，分别为－6.36～－3.39和1424～1616 Ma。研究表明，花岗细晶岩脉形成的构造背景为松潘–甘孜洋（古特提斯支洋）闭合后的后碰撞造山阶段，源岩为古老地壳变杂砂岩。结合前人研究成果，认为锂矿化花岗伟晶岩与花岗细晶岩为同一源区不同源岩（变泥质岩和变杂砂岩）部分熔融的产物。

Abstract:

The Super-large Lijiagou lithium polymetallic deposit is a newly discovered deposit in the southeastern margin of Songpan-ganzi Fold Orogenic Belt and has widespread outcrops of granite aplite veins. whole-rock geochemical analysis, zircon U-Pb dating and zircon Hf isotopic analysis for granite aplite are performed to explore its petrogenesis and geological significance. Zircons yielded weighted mean U-Pb age of 200±2 Ma for granite aplite, indicating that it was emplaced in the Late Triassic to Early Jurassic. The rocks exhibit high SiO₂ concentration (73.27%~75.14%), high Al₂O₃ concentration (14.8%~15.25%), high total alkali concentration (4.86%~8.08%) and low CaO concentration (0.45%~1.45%) with A/CNK ratios >1.1, which are defined as median-K to high-K calc-alkaline strongly peraluminous S-type granites. Trace elements of the granite aplite show positive anomalies for Rb, Th, U, K and relative negative anomalies for Ba, Sr, P, Ti. The granite aplite display enrichment of LREE, depletion of HREE, and weak negative Eu anomalies (δEu=0.68~0.81). zircon ε_Hf(t) values range from -6.36 to -3.39 with t_DM2 varying from 1424 Ma to 1616 Ma. The study shows that the granite aplite veins were formed in the post-collision orogenic stage after the closure of the Songpan-Ganzi ocean (a branch of the Paleo-Tethys Ocean), and the source rocks are the ancient crustal meta-greywacke. Combined with the previous research results, it is considered that the lithium mineralized granitic pegmatites and the granite aplites are the products of partial melting of different source rocks (meta-argillite vs meta-graywacke) in the same source area.