南秦岭印支期花岗岩带的“地幔印记”

南秦岭印支期花岗岩带的岩石组合包括闪长岩、二长闪长岩、英云闪长岩、花岗闪长岩和花岗岩等,岩体内广泛存在镁铁质微粒包体和脉体。大量(超过50%)样品具有高Mg#(<0.76),高Cr(>100×10-6,最高为1600×10-6)、Sr(>500×10-6)、Ba(>1000×10-6)的地幔印记(mantle signature)。含石榴石基性岩部分重熔模型可以解释部分样品的高LREE、低HREE和高Sr低Y特征,但无法合理解释地幔印记的存在。简单的地幔上涌、减压熔融模型虽然可以产生幔源岩浆并解释基性-酸性岩浆混合现象,但与具地幔印记样品的Sr、Nd同位素富集(ISr=0.7054～0.7085;εNd(t)=-1.52～-9.17)和区域地质特征相矛盾。具地幔印记样品与高Mg埃达克岩和太古宙sanukitoid岩系的相似性表明它们可以由含水富集地幔的直接熔融形成。

Mantle signature on the Indosinian granitoid belt in South Qinling,Central China

The Indosinian granitoid belt in South Qinling,Central China,is lithologically composed of diorite,monzodiorite,tonalite,granodiorite and granite,as well as widely distributed mafic microgranular enclaves and dykes within the intrusions. Majority (>50%) of samples have high Mg#(<0.76),high Cr(>100×10-6,max=1 600×10-6),high Sr(>500×10-6) and high Ba(>1 000×10-6) content,i. e. having the mantle signature. The partial melting of garnet bearing metabasic rocks can explain high LREE,low HREE and high Sr,low Y characteristics of some samples,but cannot explain the mantle signature. Although simple mantle upwelling and adiabatic melting model is reasonable for producing mantle-derived magma and mixing between basic and acidic magma,the enriched Sr,Nd isotope (ISr=0.705 4-0.708 5;εNd(t)=-1.52--9.17)of samples with mantle signature and the regional geological settings do not favor this model. Similarities to high Mg adakite and sanukitoid imply that the samples having mantle signature could be derived from direct partial melting of hydrous enriched mantle.