A型岩套的分类、判别标志和成因

Subdivision, Discrimination Criteria and Genesis for A Type Rock Suites

以新近在粤北开展A型岩套研究所获得的新数据,结合国内外文献中已发表的数据(17个岩体,124件样品),讨论了A型岩套亚类型划分的必要性和亚类型之间的判别方法。将A型岩套分为两亚类型三组。A_1亚类型主要以似长石正长岩、碱性正长岩和响岩为主(AS组),SiO_2不饱和、准铝、过碱-碱性岩石,通常含似长石,如方钠石、霞石、白榴石和钠沸石等为特征。A_2亚类型为SiO_2过饱和、碱质—亚碱质岩石。该亚类型又包括铝质A型花岗岩(ALAG组)(弱过铝质、亚碱质岩石)和碱性花岗岩(AAG组)(准铝质、碱质—亚碱质岩石);后者通常含少量钠钙质和钠质辉石和角闪石。在地球化学特征上,A_1亚型具低SiO_2,高ALK和NK/A值,相对富氯低氟,低R1和高R2,低Y/Nb比(具OIB源区特征),高Nb/Ta,Zr/Hf,Eu/Eu,LREE/HREE和锆石饱和温度等。A_2亚型中,以AAG组为例,其许多地球化学特征与A_1亚型相悖,如高SiO_2,较低ALK,相对贫氯富氟,高R1和低R2,高10~4Ga/Al和Y/Nb(具IAB源区特征),低Nb/Ta,Eu/Eu和LREE/HREE,以及高的锆石饱和温度等。A_2亚型中ALAG组除铝过饱和外,其他地球化学特征很相似于AAG组。利用R1-R2,Yb/Nb-Yb/Ta,Nb-Y-Ce,Nb/Ta-Nb等图解可有效地区分和判别A型岩套中A_1和A_2两种亚型。利用A_1和A_2亚型Sr,Nd同位素组成。

This paper discusses the necessity for subdivision of A type suites into A_1 and A_2 sub- types and the judging criteria to distinguish them, based on new data obtained in study on A type suites in north Guangdong province and the data published in Chinese and overseas literature. The A type suites can be divided into two subtypes and three groups (A_1 subtype: AS group; A_2 subtype: ALAG group and AAG group). The A_1 subtype (AS group) is usually SiO_2-under- saturated, metaluminous, and peralkaline-alkaline rocks, which include feldspathide syenites, al- kaline syenites and phonolites. They are essentially characterized by existence of more or less amount of feldspathide minerals such as sodalite, nepheline, leucite etc. The A_2 subtype is usually SiO_2-oversaturated, alkaline-subalkaline granitic rocks, which include the peraluminous, subalka- line A type granites (ALAG group), and the metaluminous, alkaline A type granites (AAG group). Distinguishable from the ALAG group, the AAG group generally contains more or less amount of Ca-Na- or Na-pyroxenes and amphiboles. Geochemically, the A_1 subtype has relatively lower SiO_2 (54.13%～61.29% ), higher ALK (12.65%～13.50%), NK/A (0.85～1.03), chlorine (50×10^(-6)～9600×10^(-6)), R2 (623, average, the same below), Nb/Ta (15.69), Zr/ Hf ( 36.17), Eu/Eu (0.60), LREE/HREE (15.18) and zircon-saturation temperature (833℃), and relatively lower R1 (-263), 104 Ga/Al (2.4), Y/Nb (0.34, similar to OIB source materials) and relatively lower fluorine (50×10^(-6)～3523×10^(-6)). In contrast to A_1 sub- type, the AAG group of A_2 subtype has relatively higher SiO_2(71.93%～76.28%), lower ALK (7.81%～9.80% ) , chlorine (26×10^(-6)～500×10^(-6)), R2 (318, in average, the same below), Nb/Ta (13.09), Eu/Eu (0.29) and LREE/HREE (7.57), and higher R1 (2295), 104 Ga/Al (3.93), Y/Nb (1.64, similar to IAB source materials), higher fluorine (600×10^(-6)～1900× 10^(-6)) and zircon-saturation temperature (883℃). The ALAG group of A_2 subtype is similar in geochemical properties to the AAG group, but is peraluminous. Using a series of diagrams of R1 -R2, Y/Nb - Yb/Ta, Nb - Y - Ce and Nb/Ta - Nb, the A type suites can be readily distin- guished into A_1 and A_2 subtype. Based on the Nd, Sr isotopic data and using the mixing model of depleted mantle endmember and bulk crust endmember, it is calculated that the source materials for most A_2 subtype granites of seven rock bodies are dominated by depleted mantle endmember component (68%～78% ). The mixing model of depleted mantle endmember and enriched mantle endmember shows that the source material components for A_1 type rock suites are composed of EM_4DM_(96)-EM_(17)DM_(83), and the enriched mantle endmember comprises only a minor proportion.