长江中下游成矿带火山岩盆地的成岩成矿作用

Petrogensis and Metallogeny Study of the Volcanic Basins in the Middle and Lower Yangtze Metallogenic Belt

长江中下游成矿带是我国重要的铜铁多金属成矿带,其中发育溧水、溧阳、宁芜、繁昌、庐枞、怀宁和金牛盆地等火山岩盆地。与成矿带中断隆区相比,火山岩盆地内的成岩成矿作用研究仍显滞后。本文综合已有研究工作和资料,以庐枞盆地和宁芜盆地为主,探讨长江中下游地区火山岩盆地的地质特征、岩浆岩时空格架、地球化学特征及成矿作用和成矿模式等。长江中下游成矿带火山岩盆地内岩浆岩的年代学研究表明,各陆相火山岩盆地内岩浆岩均形成于早白垩世(135～123Ma),不同盆地内各旋回火山活动时间几乎相同,盆地中侵入岩的分期、岩性和成岩时代也大致可以对比。地球化学特征研究表明,盆地内早晚两旋回火山岩与两期侵入岩在岩浆起源、岩浆演化特征、岩石成因等方面一致,可以完全对应。岩浆源区为成分接近富集的交代地幔,交代地幔的形成与古板块的俯冲交代作用有关。岩浆演化经历了结晶分异作用和同化混染作用,岩浆结晶分异作用主要发生于深部岩浆房中。长江中下游成矿带火山岩盆地中成矿作用以铁矿化为主,产出以著名的"玢岩铁矿"矿床组合为特征的一系列不同类型的火山-次火山热液型和沉积改造型等铁矿床及硫铁矿、硬石膏和明矾石矿床。矿床地质地球化学特征研究表明,铁矿床和次火山岩(闪长玢岩)关系密切,其形成还受火山构造和基底膏盐层等控制和影响。成矿年代学研究证实不同玢岩型矿床的成矿时代基本一致,均为130Ma左右,盆地内铁成矿作用是在较短时间(1～2Ma)内集中"爆发式"形成的。综合火山岩盆地成岩成矿作用特征,本文建立了长江中下游成矿带火山岩盆地的成岩成矿模式,并认为火山岩盆地成岩成矿作用是中国东部中生代构造-岩浆-成矿系统演化的有机组成部分,受中国东部中生代地球动力学背景的制约。

The Middle-Lower Yangtze River valley is one of the most important Cu-Fe polymetallic belts in China,in which hosts many volcanic basins,such as Li-Shu,Li-Yang,Ning-Wu,Fan-Chang,Lu-Zong,Huai-Ning and Jin-Niu volcanic basin.Compared with the uplift area in this region,the researches on the petrogenetic and metallogenetic in the volcanic basin are still poorly investigated.Basing on the contributions of the previous researchers as well as our fundamental studies in recent years mainly on Lu-Zong basin and Ning-Wu basin,this paper discussed the geological characteristics,the temporal-spatial framework,and geochemistry,geochronology of petrogenesis and mineralizations and metallogenetic model of the volcanic basins in the Middle-Lower Yangtze metallogenetic belt.The chronological results of the igneous rocks suggest that the magmatic rocks in the volcanic basins formed in early Cretaceous （135~123Ma）.The geochemical features of the volcanic rocks and the intrusive rocks in the volcanic basins indicate that their evolution,sources and the origin of the four formation volcanic rocks and the two epochs of intrusions are comparable.The source of the igneous rocks might be the metasomatic mantle;the formation of the metasomatic mantle was related to the subduction of the paleo-Pacific plate.The magma evolution underwent fractional crystallization and in the magma chamber and contamination during rising.Iron is the main mineralization occurred in the volcanic basins,which represented by the ＂porphyry iron deposits＂.The ＂porphyry iron deposits＂ include a series of volcanic-subvolcanic hydrothermal deposits,sedimentary-reformed deposits,pyrite deposits,anhydrite deposits and alunite deposits.The geological and geochemiscal characteristics of these deposits indicated that their formation were closely related to the subvolcanic rocks （diorite porphyry） and diorite intrusions,and also controlled by the volcanic structures and the basal halites in the Triassic strata.The mineralization chronology shows all the porphyry iron deposits are ＂booming＂ formed around 130Ma in a short interval.This paper proposes a petrogenetic and metallogenetic model for the volcanic basins in the Middle and Lower Yangtze metallogenetic belt.The petrogenetic and metallogenetic in these volcanic basins is one part of Mesozoic tectonic-magmatic-metallogenic event controlled by the geodynamic setting of eastern China in the Yanshanian period.