Microgranitoid Enclaves in Some Ⅰ-and S-type Granites from Southern China

The three l-type plutons of Guantian (GT), Guidong (GD), Shangbao (SB ) and the two S-typeplutons of Xucun (XC) and Xiuning (XN) as well as their microgranitoid enclaves in southern Chinahave been studied. Restite in the Motianling (MT) metasomatic granite in this area is described inthis paper as well. Microgranitoid enclaves in the I-type granitoids may be divided into autoliths andschlierens which have marked diffirences both in petrography and geochemistry. In the S-typegranitoids, schlierens are the major microgranitoid enclaves, but autoliths are rare. The metasomaticgranite contains only restite without other enclaves. The microgranitoid enclaves and their host rockshave close Nd (T) values and the same minerals within them are sirnilar in composition. Themicrogranitoid enclaves, in general, don’t represent the products of mjxing of the syn-plutonic foreignmafic magna and the host acidic magma. They are the reeords of the evolution of intermediate-acidicmagma itself The formation of autoliths is related to the interdiffosion of different conshtuents inmagma. Schlierens are the products of immiscible fractionation of the magma.

Morphology and geochemistry of zircon: a case study on zircon from the microgranitoid enclaves

There are three types of zircon (i.e. Zircon A, Zircon B and Zircon C) in the microgranitoid enclaves from the Qingtian granite. Zircon A is of the smallest Ipr, Ipy and Iel values with the largest range of variations; Zircon C is of the largest Ipr, Ipy and Iel values with the smallest range of variations; and Zircon B is intermediate among the three types. The microprobe analysis of zircon demonstrates that the contents of trace elements (Hf, U, Y, Th) increase progressively with larger and larger variation from Zircon A through Zircon B to Zircon C. These characters snggest that the three types of zircon in the enclaves may have formed successively during the cooling mess of enclave magma, corresponding to different sites along with the intrusion of enclave magma. Because of positive correlations of the UO2/HfO2ratio with Ipr, the Th02/Y203 ratio with Ipy, and the UO2/(ThO2+Y203) ratio with Iel, it is suggested that the variation in zircon typology is caused by selective substitution of trace elements on different surfaces of zircon. On the basis of enclave petrology and zircon typomorphism, it is believed that the enclave magma may be generated from partial melting of residual materials in deep regions where the granitic magma has been extracted.

小兴安岭东南金山屯一带晚三叠世二长花岗岩成因及其地质意义

在小兴安岭东南端金山屯一带,广泛发育近南北向展布的晚三叠世似斑状二长花岗岩岩石组合,岩石中普遍见有暗色微细粒闪长质包体和中—基性脉岩群。对岩体及其包体、中—基性脉岩的宏观、微观特征和地球化学特征的研究表明,包体形态多呈浑圆的外形,显示出明显的塑性流变特点,具典型的岩浆结构包体中见针状磷灰石和含寄主岩钾长石、石英巨晶,具有明显的岩浆混合成因的包体特征;岩体的中—基性脉岩(群)形态多样,与花岗岩的界面或呈小波浪状或呈平直状,并见寄主岩长石斑晶,表现出壳幔混合作用形成的同深成岩墙群特征。该区晚三叠世二长花岗岩具显著的岩浆混合成因特征,其形成可能与碰撞后伸展动力学机制下的基性岩浆底侵作用有关。

花岗岩中的钾长石巨晶:以南岭佛冈花岗质杂岩体中微斜长石巨晶为例

野外地质观察和岩石显微结构研究表明 ,佛冈花岗质杂岩体中微斜长石巨晶是岩浆结晶的产物 ,不是交代斑晶 ,也不是变斑晶。它们与基质中的微斜长石构成双峰式粒径 ,反映了岩浆的两阶段结晶历史。由于微斜长石是钾长石的低温变体 ,因此一种可能的机制是佛冈花岗质杂岩岩浆在侵位和基本固结后 ,冷却缓慢 ,使早结晶的正长石转变为微斜长石。此外 ,在新近的研究中 ,还发现了罕见的、但在佛冈花岗质杂岩体中为数不少的“十字”贯穿式双晶微斜长石 ,以及微斜长石巨晶的“环斑”和“珠边”结构。

寄主花岗岩对微花岗岩类包体的地球化学制约

本文报道了华南Ｉ型关田、上堡岩体及Ｓ型许村、休宁岩体中微花岗岩类包体的特征。Ｉ型花岗岩中的微花岗岩类包体分为同源包体和析离体，两者在岩相学及地球化学上有明显区别。在ｓ型花岗岩中微花岗岩类包体的主要类型是析离体，同源包体则很少出现。微花岗岩类包体及其寄主岩具有相接近的εＮｄ（Ｔ）值，两者中同种矿物的成分相近似。寄主花岗岩在矿物地球化学上对微花岗岩类包体有明显的制约。微花岗岩类包体一般不是外来基性岩浆与寄主岩浆相混合的结果。同源包体的形成与中酸性岩浆中不同组分的相互扩散有关，析离体则是岩浆熔离作用的产物。

微花岗岩类包体与岩浆的扩散作用和熔离作用——以诸广山桂东、上堡岩体为例

诸广山加里东期桂东及上堡黑云母花岗闪长岩体中的微花岗岩类包体包括同源包体和析离体。同源包体与寄主岩的暗色矿物组成不同,主要含角闪石;析离体与寄主岩中的暗色矿物组成相同,主要含黑云母。包体与寄主岩的矿物、岩石化学、地球化学及产状特征对比表明,这两种包体的形成涉及到岩浆内部组分的相互扩散、岩浆的熔离、对流及派生岩浆的相互混合。形成微花岗岩类包体的偏基性岩浆大多数是中酸性岩浆自身演化的产物。微花岗岩类包体的形成是岩浆结晶过程的记录。

小兴安岭东南晚奥陶世鹤林侵入杂岩体成因探讨

小兴安岭东南晚奥陶世鹤林侵入杂岩体岩性变化较大,由角闪辉长岩—(石英)闪长岩—英云闪长岩—二长花岗岩组成,不同岩石类型在野外宏观上呈相互侵入、包裹和渐变的接触关系;在(石英)闪长岩、英云闪长岩和二长花岗岩中均发育微细粒闪长质包体,包体多具明显塑性流变特点的浑圆外形和典型岩浆结构、针状磷灰石及捕获的寄主岩钾长石、石英斑晶,为MME型岩浆混合成因包体;杂岩体相对富集LILE(Ba、Sr、Rb)、HFS(U、Th、Zr、Ce)元素等,显示出壳源特点,而较高的εNd(t)值(-2.38^-3.77)显示以幔源为主的特点。研究表明杂岩体具明显的壳幔岩浆混合成因的岩相学、岩石化学和地球化学特征,形成于基性岩浆底侵作用下的陆缘弧型活动大陆边缘构造环境。