Petrogenesis of magmatic albite granites associated to cogenetic A-type granites: Na-rich residual melt extraction from a partially crystallized A-type granite mush
Petrogenesis of magmatic albite granites associated to cogenetic A-type granites: Na-rich residual melt extraction from a partially crystallized A-type granite mush
复制标题
DOI：
10.1016/j.lithos.2013.07.005
复制
发表时间：
2013-09
期刊：
影响因子：
3.5
通讯作者：
M. Barboni;F. Bussy
中科院分区：
文献类型：
--
作者：
M. Barboni;F. Bussy
在法国布列塔尼的347 Ma双峰式Saint-Jean-du-Doigt（SJDD）岩体中发现了一种罕见的同源和近同期钾长石A型花岗岩和钠质钠长花岗岩组合。A型花岗岩以细粒、粉红色至黄色岩石的小岩体（< 1平方公里）或镁铁质层之间一米厚的岩床形式出露。它们很早就侵位在SJDD岩体的热“冷”部分，直接位于前寒武纪寄主岩石之下，形成了岩体顶板。钠长花岗岩是在岩体热成熟区稍晚于A型花岗岩侵位的细粒全淡色淡黄色岩石。它们形成了米厚的岩床，与相邻的镁铁质层混合在一起，代表了约1000万年前。1体积%两种花岗岩类型在许多方面具有相似性，Sr-Nd-Hf同位素组成相当A型花岗岩的~（87）Sr/~（86）Sr ~（347）= 0.7071，钠长花岗岩的~（87）Sr/~（86）Sr ~（347）= 0.7073，ε Nd ~（347）= + 0.2，钠长花岗岩的~（86）Sr/~（347）=+ 0.3; ε Hf 347锆石=+2.47 vs.+2.71）和SiO2含量（74.8 vs. 74.4 wt.%）。另一方面，它们在K2 O中具有对比浓度（5.30对1.97重量%），Na 2 O（2.95与4.73重量%）和CaO（分别为0.48和2.04）以及一些微量元素，如Sr（平均59和158 ppm）、Rb（87和35 ppm）、Cr（170和35 ppm）和Ga（30和20 ppm）。A型花岗岩和钠长花岗岩的同位素组成与伴生的体积占优势的镁铁质岩石（87 Sr/86 Sr 347 = 0.7042; ε Nd 347 = + 5.07; ε Hf 347锆石= + 8.11）明显不同，排除了长英质岩石是由玄武质岩浆分离结晶而成的。另一方面，少量的混合，包体轴承花岗闪长岩内的SJDD岩片建议混合过程中位于地壳较深的水库。因此，A型花岗岩可能是镁铁质岩浆与地壳熔体混合形成的。或者，它们可能来自于未成熟的含黑云母的石英-石英质地壳原岩的纯熔融，该原岩是由早期低地壳水平的镁铁质注入引起的。锆石Sr、Nd、Hf同位素、锆石U-Pb年龄、岩相学、矿物化学和元素地球化学等资料表明，A型花岗岩与钠长花岗岩关系密切。我们首选的成岩模型是考虑钠长石花岗岩岩浆作为一个组成的极端熔体，是从部分结晶的A型花岗岩糊状物在结晶的后期阶段提取。钠长花岗岩可能是由A型花岗岩分异过程中形成的富斜长石层熔融而成。
The uncommon association of cogenetic and nearly contemporaneous potassic K-feldspar A-type granites and sodic albite granites is observed within the 347 Ma-old bimodal Saint-Jean-du-Doigt (SJDD) intrusion, Brittany, France. A-type granites outcrop as small bodies (< 1 km2) of fine-grained, pinkish to yellowish rock or as meter-thick sills in-between mafic layers. They emplaced early within the thermally “cool” part of the SJDD pluton directly beneath the Precambrian host rock, forming the pluton roof. Albite granites are fine-grained hololeucocratic yellowish rocks emplaced slightly after the A-type granites in the thermally mature part of the pluton. They form meter-thick sills that mingle with adjacent mafic layers and represent ca. 1 vol.% of the outcropping part of the pluton.The two granite types are similar in many respects with comparable Sr–Nd–Hf isotope compositions (87Sr/86Sr347= 0.7071 for A-type granites vs. 0.7073 for albite granites; εNd347= + 0.2 vs. + 0.3; εHf347zircon= + 2.47 vs. + 2.71, respectively) and SiO2contents (74.8 vs. 74.4 wt.%). On the other hand, they have contrasting concentrations in K2O (5.30 vs. 1.97 wt.%), Na2O (2.95 vs. 4.73 wt.%) and CaO (0.48 vs. 2.04, respectively) as well as in some trace elements like Sr (59 vs. 158 ppm in average), Rb (87 vs. 35 ppm), Cr (170 vs. 35 ppm) and Ga (30 vs. 20 ppm). The isotopic composition of the A-type and albite granites is very distinct from that of the associated and volumetrically dominant mafic rocks (i.e.87Sr/86Sr347= 0.7042; εNd347= + 5.07; εHf347zircon= + 8.11), excluding a direct derivation of the felsic rocks through fractional crystallization from the basaltic magma. On the other hand, small volumes of hybrid, enclave-bearing granodiorite within the SJDD lopolith suggest mixing processes within a reservoir located at deeper crustal levels. A-type granites may therefore form by magma mixing between the mafic magma and crustal melts. Alternatively, they might derive from the pure melting of an immature biotite-bearing quartz-feldspathic crustal protolith induced by early mafic injections at low crustal levels.Strong field evidences coupled to mineral chemistry and elemental geochemistry strongly support a magmatic origin for the albite granite. Sr, Nd, Hf zircon isotope data, U–Pb zircon ages, as well as data on petrography, mineral chemistry and elemental geochemistry attest that A-type and albite granites are closely related. Our preferred petrogenetic model is to consider the albite granite magma as a compositionally extreme melt that was extracted from a partially crystallized A-type granite mush at a late stage of crystallization. Alternatively, albite granites could form by melting of plagioclase-rich layers formed during A-type granite differentiation.