皖南中生代基性岩：源区性质和构造背景

Yanshanian magmatic rocks are broadly cropped out in the southern Anhui Province, the lower Yangtze region with rock-type of dominant granitoids. As yet, some degrees of studies on formed ages and geochemical characteristics of these magmatic rocks, as well as petrogenesis and their tectonic setting have been performed. However, to be the key status in magmatic activities, characteristics of the lithospheric mantle and its contributions in magmatisms are still ambiguous. It is still poorly understood that whether and when the lithospheric mantle was metasomatized by subducted materials. Moreover, there is hot debate about the tectonic setting of magmatisms concerning intraplate orogen or active continental margin, or their transformation corresponding different stages of magmatic activities. The key accounting for these issues depends on detailed constraints on characteristics of the Mesozoic lithospheric mantle and its formation mechanism. Based on new found five places of Mesozoic basic rocks and peridotite xenoliths during field survey in the southern Anhui province, integrated studies of chronology, petrology, mineral in situ major and trace elements, whole rock geochemistry, and zircon in situ Hf-O isotopes will be carried out, aimed to deciphering petrogenesis and magmatic process, revealing petrological and geochemical characteristics of the lithospheric mantle, analyzing formed mechanism and age of the potential enriched signatures of the lithospheric mantle, as well as constraining the petrogenesis of the granitoids. Then, the Mesozoic continental tectonic setting and its evolution in the southern Anhui province will be further constrained.

下扬子皖南地区燕山期岩浆作用强烈，岩石类型以花岗岩类为主。目前，对岩浆岩时代、地化特征、成因和构造背景有着一定程度的认识，但对于在岩浆作用中处于关键地位的岩石圈地幔的性质及其在岩浆活动中物质上的贡献还不清楚，对岩石圈地幔是否和何时受到俯冲物质的交代也缺乏认识，对岩浆作用的构造背景是板内造山还是活动大陆边缘，或者是对应不同岩浆活动阶段存在两种构造背景的转换等有着激烈争议。探索这些问题的关键是对中生代岩石圈地幔性质及其形成机制的深入研究。本项目基于在皖南野外调查新发现的5处中生代基性岩和橄榄岩包体，拟开展年代学、岩石学、矿物原位主量和微量元素、全岩地球化学以及锆石原位Hf-O同位素的系统研究，深入辨析岩石成因和岩浆过程，揭示岩石圈地幔的岩石学和地球化学特征，剖析岩石圈地幔可能存在的富集性质的形成机制和形成时代，同时对花岗岩类的成因给予进一步制约，进而确定皖南地区中生代大地构造背景及其演化过程。

下扬子地区广泛发育中生代岩浆岩，同时伴随着大规律多金属成矿作用，但是对该时期地幔源区的性质以及深部过程的认识还不清晰，对相应的构造背景和演化的认识还存在激烈争议。本次工作针对皖南地区6个晚中生代基性岩体、十多个晚中生代花岗岩体，以及蚌埠地区晚中生代花岗岩和基性岩体开展了岩石学、全岩地球化学、锆石原位Hf同位素以及锆石年代学的系统研究，揭示了江南造山带东段晚中生代岩石圈地幔具有弱富集的性质（εNd(t)=-1），受到了俯冲大洋板片析出熔体的交代，且在~125 Ma时期，发育有起源于金红石分解深度的幔源岩浆，指示了江南造山带东段晚中生代从宽阔陆源弧转化为大地慢楔构造环境，转变时期为125 Ma；建立了皖南中生代岩浆作用三阶段精细年代学格架（152~137 Ma， 136~129 Ma，128~123 Ma），确定了岩浆岩成矿专属性，多金属成矿主要与第一阶段岩浆作用成因相关；确定了池州地区晚中生代岩浆岩岩石成因及成矿规律，长江中下游地区Cu-Au成矿与幔源岩浆成因相关，而江南造山带东段W-Mo成矿与中—新元古代增生地壳活化成因相关；确定了蚌埠地区淡色花岗岩岩石类型及其成因，揭示了印支期俯冲的华南陆壳在华北东南缘燕山期岩浆作用中的物质贡献及其对成矿的意义。本次工作成果的意义在于将更加清晰和深入地认识中国中东部燕山期成岩成矿作用及其大地构造动力学背景的演化。