东安金矿光华组酸性火山凝灰岩地球化学、锆石U-Pb年龄和Hf同位素特征及其地质意义

Geochemistry,U-Pb Geochronology,and Hf Isotopic Characteristics and Geological Significance of the Tuff from Dong'an Gold Deposit,Lesser Khingan Mountains,NE China

东安金矿位于松辽盆地北部边缘,早白垩世拉张断陷作用强烈,浅成侵入体和火山机构发育。样品岩石地球化学分析表明,岩体高硅、富碱、贫钙和镁,属于高钾钙碱性、过铝质系列酸性火山凝灰岩。岩体明显富集Rb、K等大离子亲石元素和Th、U等活泼的不相容元素,强烈亏损P、Ti、Nb、Ta等高场强元素;轻、重稀土元素分馏明显,(La/Yb)N=6.81~23.24,有轻微负Eu异常(δEu=0.34~1.02)。岩浆锆石LA-ICP-MS U-Pb年龄分别为107.4±2.0 Ma(MSWD=2.0)和178.7±5.1 Ma(MSWD=3.2),分别属于早白垩世和中侏罗世。锆石的原位微区Hf同位素εHf(t)=+2.48^+7.08,176Hf/177Hf比值0.282 762~0.282 857,二阶段模式年龄TDM2=693~897 Ma。岩浆源区主要为来源于新元古起源于亏损地幔的变质玄武岩类,同时有古老地壳物质的加入。岩石形成于白垩世晚期太平洋板块俯冲引起的中国东北部大陆弧伸展和岩石圈减薄时期。区内浅成低温成矿作用发生在火山机构演化的晚期,成岩成矿受控于统一的构造背景。

Dong＇an gold deposit is located in Songliao Basin. This area underwent tensional faulting and subsidence associated with magmatism during Early Cretaceous,and formed numerous shallow intrusions and volcanic edifices. The dominant wall rock is peraluminous to metaluminous volcanic rocks. Samples have high concentrations of large ion lithophile elements and incompatible elements;strongly depleted in high field-strength element,light and heavy rare earth elements are clearly fractionated and have slightly negative Eu anomalies（ δEu = 0. 45 ~ 1. 02）. All geochemical characteristics suggest that the tuff were formed in an arc environment. Zircons U – Pb dating yielded weighted average ages of 107. 4 ± 2. 0 Ma（ MSWD = 2. 0） and178. 7 ± 5. 1 Ma（ MSWD = 3. 2）. In situ zircon Hf isotope analysis yielded εHf（ t） values between ＋ 2. 48 and ＋ 7. 08,176 Hf /177 Hf ratios of 0. 282 762 ~ 0. 282 857,and two-stage model age of 693 ~ 897 Ma,suggesting the tuff formed from magmas derived from metamorphosed basalts originally sourced from a depleted region of the mantle that interacted with ancient crustal material. The tuff formed during extension of continental arc and thinning of the lithosphere,which were caused by subduction of the Pacific Plate during the late Early Cretaceous. Mineralization of the deposit took place after digenesis of tuff. Epithermal mineralization within the study area formed during the later stages of volcanic evolution,and structural features controlled the locations of both mineralization and diagenesis. Epithermal mineralization was closely related to intrusive magmatism in the eastern of Jilin and Heilongjiang Province.