淡色花岗岩的岩石学和地球化学特征及其成因

Petrological and geochemical constraints on the origin of leucogranites

淡色花岗岩(leucogranite)是一类高铝高硅碱的酸性侵入岩,主要地球化学特征是:SiO2≥72%,Al2O3≥14%,Na2O+K2O^8.5%,富Rb,亏损Th、Ba、Sr,稀土总量较一般花岗岩低(∑REE=(40~120)×10-6),且表现为中等分异的轻稀土弱富集型,一般具有Eu负异常;Sr-Nd-Pb-O同位素指示其岩浆明显的陆壳来源。淡色花岗岩主要发育于陆壳(俯冲)碰撞加厚带,由逆冲折返的俯冲板片变沉积岩部分经过脱水熔融产生。淡色花岗岩可划分为三种不同的岩石类型:(1)二云母型淡色花岗岩,由变泥质岩(或变硬砂岩)在中地壳水平经黑云母(和/或白云母)脱水熔融产生;(2)电气石型淡色花岗岩,由变泥质岩在较低温度下经白云母脱水熔融产生;(3)石榴子石型淡色花岗岩,由长英质下地壳经黑云母脱水熔融产生。源区残留独居石、磷灰石等富REE矿物是淡色花岗岩亏损REE、Th等元素的原因。源岩为变泥质岩及源区残留钾长石是淡色花岗岩亏损Sr、Ba的主要原因。

Leucogranite is a strong peraluminous felsic intrusion （with SiO2 ≥72%） linked to anatexis of continental crust. It is rich in aluminium （Al2O3≥14%）, alkali elements （Na2O＋K2O～8.5%） and enriched in Rb, and depleted in Th, Ba, Sr. Their ∑REE contents （mostly between 40× 10^-6-120× 10^-6） are lower than those of other granites and show slightly LREE enriched patterns and a negative Eu anomaly. The Sr-Nd-Pb-O isotopic compositions of leucogranites suggest that they were derived from continental crust by partial melting. Leueogranites are commonly found in deformed metapelite/metagraywacke sequences that have thrust over basement, during crustal thickening associated with continental collisions. There are three types of leucogranires with different petrogenesis： （1） two-mica leucogranlte, which is formed by biotite and/or muscovite dehydration melting of metapelite （or metagraywacke） in middle crust; （2） tourmaline leucogran/te, which is formed by muscovite dehydration melting of metapelites at lower temperature; （3） garnet-bearing leucogranite, which is most likely generated by biotite dehydration melting of felsic lower crust. The REE enriched minerals, such as monazite and apatite, as the residue phases in magma source could be responsible for depletions of REE and Th in leucogranite. Metapelite as the source rock or K-feldspar as residue phases in magma source could be responsible for depletions of Sr and Ba in leucogranite.