浙江小將-梁弄岩體及其包體之礦物化學與地球化學研究

Abstract

華南地區晚燕山期侵入岩漿活動，廣布晚侏儸紀至早白堊紀的花崗岩類，主要發育於浙、閩沿海地區，即東南沿海岩漿活動帶上，包括大量I型花崗岩類、及少量的A型花崗岩的出露並伴隨雙模式火山活動的產物，本文除了以浙江小將、梁弄兩岩體之主岩及其包體為主要分析目標外，相關討論另包括浙江地區龍王堂、山頭鄭、桃花島、青田及福建地區福州、漳州、魁岐、筆架山等岩體標本，皆屬於華南晚燕山期花崗岩類的範疇。 本研究小將岩體之包體標本共包括三類包體，由礦物化學與地球化學分析結果顯示，小將岩體的第一類包體與主岩相似，兩者間具有結晶分化的關係，包體部分可能來自早期冷凝的基性組分(基性冷凝邊)，形成同源包體，而第三類包體具有富鎂、貧鈣的特徵，則為第一類組合早期結晶出來的基性產物。第二類包體屬於中鉀系列，相對於其他包體而言，具有低鉀、高鈉、高鋁的特徵，其Sr同位素及地化特徵，顯示為後造山型岩漿活動特徵，其來源可能為後造山一開始底侵的基性岩漿早期侵入所成的岩類，後被高結晶分化的主岩所帶至淺處。本研究梁弄岩體標本只含一類包體，顆粒大小與主岩一致，所有的標本分析呈現一致或漸變的現象，並顯示高溫至低溫的結晶溫度，其間具有結晶分化的關係，包體屬於分化的產物。由礦物化學分析結果顯示，礦物的結晶狀態處於低溫低壓，並由高水壓轉變至低水壓的環境；另利用角閃石地質壓力計，求得岩漿侵入時的深度約為3-8公里，與福建福州、漳州複式岩體研究結果相符，皆說明本研究標本屬於淺侵入的生成環境。 小將-梁弄岩體的分析結果與浙閩地區晚燕山期部分花崗岩類的比較結果，其侵位年代、侵位空間分佈、地化特徵皆十分相近，同屬於晚燕山期後造山型的I-A型花崗岩，顯示為擠壓到張裂的生成環境。

Late Yanshanian magmatism in South China caused vast granitic intrusions including large amounts of I-type granitoids and small amounts of A-type granites from late Jurassic to early Cretaceous, distributed mainly in the Southeast Coast Magmatic Belt (SCMB). The Xiaojiang-Liangnong Complexes and their enclaves in Zhejiang is the target of this study, samples from Longwangtang, Taohua Island, Qingtian in Zhejiang and Fuzhou, Zhangzhou, Kuiqi, Bijiashan in Fujian are concerned. Xiaojiang Complexes include three types of enclaves and two types of hosts. The analyses of mineral chemistry and geochemistry indicate that characters of the first type of enclaves are similar to their hosts, which could have resulted from differentiation of the enclaves representing fragments of early, chilled margins, disrupted during the ascent of magmas. The third type of enclaves are characterized by high-Mg, low-Ca compositions, and they represent the basic products of early differentiation from magmas of the first type. The second type of enclaves belonging to medium-K series are characterized by high-Al,Na compositions. The Sr isotope and geochemical data indicate they could differentiate from the underplating basic magmas of post-orogenic stage of Late Yanshanian magmatism. Liangnong Complexes include one type of enclaves and two types of hosts. All samples from Liangnong Complexes belong to products of differentiation. Using the Al-in-amphibole geobarometer, the samples of this study were estimated to have been emplaced at shallow depths (3-8 km). The analyses results of Xiaojiang-Liangnong Complexes are close to the emplacement ages, depths, geochemical characters of the Late Yanshanian granitoids in Zhejiang and Fujian in this study. They all belong to the intrusions of I- and A-type granitoids of post-orogenic stage of Late Yanshanian magmatism reflecting tectonism from compressional to extensional environment.