新疆东天山哈尔里克二叠纪奥莫尔塔格碱性花岗岩特征、成因及构造意义

Characteristics, petrogenesis and tectonic implications of the Permian Omoertage alkaline granites in Harlik area, Xinjiang

新疆哈尔里克山东段发育的奥莫尔塔格碱性花岗岩岩体侵位于小铺东花岗岩中,岩体岩性较为均一,呈紫红色,含有典型的碱性镁铁矿物镁钠铁闪石和霓石,副矿物有锆石、钛铁矿以及萤石等。化学成分上,该岩体富硅、碱、铁,贫钙、镁,低铝,富Rb、Th、Cs、U、Zr,贫Ba、Sr、P、Ti等,Nb、Ta、Hf和Ga等元素的含量也较高,104Ga/Al值变化于3.41~3.65之间。稀土元素总量较高,配分模式右倾,并具明显的Eu负异常(δEu=0.30~0.33);具有较高的锆饱和温度(940~952℃,平均为947℃)。锆石LA-ICP-MS定年测得代表性样品的U-Pb年龄为288.9±1.6 Ma,表明该岩体属于早二叠世岩浆活动的产物。奥莫尔塔格碱性花岗岩具有偏高的εHf(t)值(+8.7^+12.5)和较年轻的单阶段模式年龄(600~430 Ma),并以具较高的Y/Nb值(2.92~3.17)特征类似于A2型花岗岩。结合区域构造背景和岩体的地球化学特征分析,认为奥莫尔塔格A2型碱性花岗岩是新生地壳发生部分熔融形成相当于英云闪长岩、花岗闪长岩等钙碱性的岩石后,它们作为源岩发生再次部分熔融的产物。

Located in the east segment of Harlik, the Omoertage alkaline intrusion （OMTG） was emplaced in Xiaopudong calc-alkaline granitic pluton during the Late Paleozoic. The purplish red OMTG alkaline granites exhibit uniform micrographic texture and contain typical alkaline ferromagnesian minerals such as arfvedsonite and aegirine and accessory minerals like zircon, ilmenite and fluorite. Geochemically, the alkaline granites are rich in silicon, alkalis, iron, and poor in calcium and magnesium, and show enrichment of Rb, Cs, Th, U and Zr and depletion of Ba, Sr, P and Ti. They also have relatively high concentrations of Nb, Ta, Hf, Ga, and display high （104 Ga/Al） values （3.41~3.65） and strong Eu depletions （δEu=0.30~0.33）. Zircon saturation temperatures of OMTG alkaline granites vary from 940 to 952℃, which is in agreement with the characteristics of typical A-type granite. Zircon LA-ICP-MS U-Pb dating yielded a mean age of 288.9±1.6 Ma, indicating that the alkaline intrusion was formed during the Early Permian. The OMTG granites have high εHf（t） values （＋8.7~＋12.5） and young one-stage Hf model ages （600~430 Ma）. The alkaline granites also show higher Y/Nb ratios （2.92~3.17）, suggesting that they belong to the A2 type granite. An integrated study of the tectonic setting and geochemical characteristics show that the OMTG A2-type granite experienced fractional crystallization by the alkaline magma which was produced by the partial melting of Neoproterozoic-Early Paleozoic juvenile crust induced by slab break-off after post-collision events.