苏门答腊中部Panti早始新世S型花岗岩地球化学特征及其新特提斯构造意义

Geochemical Characteristics of the Early Eocene S-type Granites in Panti, Central Sumatra and its Neotethyan Tectonic Implications

苏门答腊地处特提斯构造域与印度洋岛弧系统之内,区内广泛发育中‒新生代火成岩。这些火成岩对特提斯构造带的延伸及界定具有重要意义,但目前对于苏门答腊岛的研究程度较低。本文选取苏门答腊岛中部Panti地区的花岗岩进行了岩相学、锆石年代学、锆石原位Hf同位素和全岩地球化学研究。年代学研究表明,Panti花岗岩锆石U-Pb年龄为55.5±0.3 Ma。该套花岗岩具有高SiO2(76.61%~78.37%)和高碱(Na_(2)O+K_(2)O=7.47%~8.21%)的特征,其分异指数(DI)为95~96,A/CNK为1.08~1.12,为高分异S型花岗岩。稀土配分模式图及微量元素蛛网图显示其具有强烈的Eu负异常(Eu/Eu^(*)=0.04~0.05),并亏损Nb、Ta、Sr、P和Ti的特征。且该套花岗岩还具有高Rb/Sr和低CaO/Na_(2)O值,同时显示富集的锆石Hf同位素组成(εHf(t)=−9.1~−2.6)和较老的Hf二阶模式年龄(tDM2=1.29~1.70 Ga),这些特征均指示该套花岗岩来源于古老的变泥质岩的部分熔融。对比缅甸‒滇西‒藏南一带同时期的岩浆事件,我们认为Panti早始新世高分异S型花岗岩可与滇西腾梁和东盈江同时期花岗岩进行对比,指示了新特提斯构造岩浆带南延至苏门答腊中部。

The Sumatra Island,which is located in the tectonic junction of the Tethyan domain and the Indian Ocean subduction zone,has widespread outcrops of Meso-Cenozoic igneous rocks.These igneous rocks are important to constrain the extension of the Tethys tectonic belt in Sumatra,but remain poorly studied.In this paper,we present a set of petrographic,zircon U-Pb geochronological,zircon in-situ Hf isotopic,and whole-rock geochemical data for the granites from the Panti area in the central Sumatra.LA-ICP-MS zircon U-Pb dating showed that the Panti granites were crystallized at 55.5±0.3 Ma.The Panti granites have high SiO2(76.61%–78.37%),high alkali(Na_(2)O+K_(2)O=7.47%–8.21%),differentiation index(DI)(95–96),and A/CNK(1.08–1.12)values,indicative of highly fractionated S-type granite.The granites are characterized by significant negative Eu anomalies(Eu/Eu^(*)=0.04–0.05)and depletion of Nb,Ta,Sr,P,and Ti.The high Rb/Sr and low CaO/Na_(2)O ratios,along with the negativeεHf(t)values(−9.1 to−2.6)and old tDM2 ages(1.29–1.70 Ga)indicate that the granites were derived from partial melting of ancient pelitic-rich metasediments.The early Eocene granites in the Panti area have geochemical features similar to those of the Tengliang and Yingjiang areas in Western Yunnan.Therefore,it is suggested that the Neotethyan belt southerly extend to the central Sumatra.