安徽庐枞盆地黄屯地区岩浆侵入活动时限及元素地球化学特征

庐枞矿集区是长江中下游成矿带重要的铜铁多金属矿集区之一,近期勘查工作在矿集区北部的黄屯地区取得找矿突破,发现了数个与岩浆侵入活动有关的铅锌铜金矿床。对安徽庐枞盆地黄屯地区的侵入岩开展了较为系统的LA-ICP-MS锆石U-Pb定年及全岩地球化学分析,旨在确定黄屯地区岩浆侵入活动时限和揭示岩浆演化过程。结合前人年代学数据及侵入体之间的接触关系,明确黄屯闪长玢岩最早侵位(135.0～134.4 Ma),岳山二长斑岩次之(132.7～132.1 Ma),焦冲正长岩最晚形成(131.5～129.6 Ma),均属于庐枞盆地内部砖桥旋回末期岩浆侵入活动的产物。黄屯地区的侵入岩具有Si近饱和、富Al、高碱、富K、低Ti的特征,富集Rb、K等大离子亲石元素,亏损Nb、Ta等高场强元素,为典型的橄榄玄粗岩系列。结合Sr-Nd同位素特征,推测岩浆源区可能为富集型扬子板块岩石圈地幔。主量、微量元素的演化趋势显示,矿物分离结晶作用控制了本区岩浆演化过程,从闪长质岩浆至二长质岩浆主要发生角闪石的分离结晶作用,而从二长质岩浆至正长质岩浆则主要发育斜长石的分离结晶作用。

Geochronology and Geochemistry Characteristics of Magmatic Intrusive Rocks in Huangtun Area of Luzong Basin,Anhui, China

Luzong ore district located at northern margin of Yangtze block is one of important iron-copper polymetallic ore districts in the middle-lower Yangtze river metallogenic belt. The porphyry-type iron and copper deposits are the dominant types in Luzong ore district. Recently, several medium-sized lead-zinc-copper-gold deposits, which are genetically related with regional magmatic intrusive rocks, are discovered in Huangtun area, the northern margin of Luzong Basin, Anhui. Petrological observations, LA-ICP-MS zircon U-Pb dating and geochemical analysis were carried out for intrusive rocks in Huangtun area with the aims to determine the temporal framework of magmatic intrusions and to reveal the magmatic evolution process. Huangtun dioritic porphyrite, Yueshan monzonitic and Jiaochong syenitic intrusive rocks form at 135.0-134.4, 132.7-132.1 and 131.5-129.6 Ma, respectively, which belong to the products of intrusive magmatism at the end of Zhuanqiao volcanic cycle in Luzong Basin. The intrusive rocks in Huangtun area are characterized by high SiO₂(mean content is 61.67%), Al₂O₃(16.95%), alkaline(10.25%)and low TiO₂(0.66%), enriched in large ion lithophile elements such as Rb, K as well as light rare earth elements, and depleted in high strength field elements such as Nb and Ta. Geochemical features suggest that the intrusive rocks belong to typical shoshonite series. Combined with Sr-Nd isotopic compositions, the primary magma is probably derived from enriched lithospheric mantle of Yangtze block. Geochemical variations reveal that fractional crystallization controls the evolution process of the magma, during which the fractionation of hornblende dominates the differentiation from diorite to monzonite, and plagioclase dominates the fractional crystallization from monzonite to syenite.