The Xuhe mafic rocks, located in Ziyang county of Shaanxi Province, are dominated by diabase-porphyrite, gabbro–diabase, diabase, and pyroxene diorite. Primitive mantle-normalized multi-element patterns show that, the Xuhe mafic rocks are enriched in large ion lithophile elements(LITE), such as Ba and Pb, depleted in K and Sr for basic rocks, and are depleted in Sr, P and Ti for pyroxene diorite. Chondrite-normalized REE patterns display LREE enrichment(LaN/YbN = 9.34–13.99) and have normalized patterns for trace element and REE similar to that of typical OIB. Detailed SIMS zircon U–Pb dating yields emplacement ages of 438.4 ± 3.1 Ma for Xuhe mafic rocks. The relatively low Mg O(basic rock: 3.11–7.21 wt%; pyroxene diorite: 0.89–1.21 wt%) and Mg#(0.20–0.49) for Xuhe mafic rocks suggest that they were possibly originated from an extremely evolved magma. The rising parental mafic magmas underwent pyroxene and plagioclase fractionation. Crustal contamination of pyroxene diorite before emplacement occurred at a higher crustal level compared to other lithology in Xuhe mafic rocks. The degree of partial melt was low(5%–10%) and in garnetspinel transition facies. Sr-Nd isotope of pyroxene diorite and enrichment mantle characteristics for Xuhe mafic rocks suggest that mafic rocks in the North Daba Mountains were derived from a mixture of HIMU, EMII and small amount of EMI components. Furthermore, this study discusses mantle geodynamic significance of Xuhe mafic rocks in the Silurian, which indicates subduction and uplift of magma caused back-arc extension.
The tectonic evolution of South Qinling,which is a main part of the Qinling orogenic belt,is still in dispute and deformation history of South Qinling is poorly studied.In this paper,detailed structural,microstructural,quartz c-axis fabric analysis,and geochronology results for the Madao gneiss in South Qinling are presented to characterize the deformation history.Results show that rocks in the northern part(Tiefodian-Laozhanggou) experience general shearing and deform at relative low temperature.The shear sense generally is south to north.In contrast,rocks in the southern part(Laozhanggou-Panjiahe) are weakly sheared with pure shear features and evidence of hightemperature deformation.Based on the analyses,we conclude that there exist two distinct deformation geometries in the Madao gneiss and accordingly we can divide the deformation into two stages.The early stage is represented by regional shortening,while the late stage features northward thrust shearing and evidence shows that it was a progressive process between them.LA-ICP MS U-Pb dating of zircons from pre-deformational migmatite veins yields age of 198.5 ± 2.0 Ma.This result,in combination with the age of post-deformational granite,indicates that the northward thrust shearing of the Madao gneiss occurred in the Late Triassic.In view of these results and other reported data in South Qinling,we propose that deformation in Madao gneiss may result from the initial collision and subsequent northward accretion in Late Triassic.
WANG DongshengWANG ZongqiZHANG YingliWANG TaoCHEN LeiZHANG Yutao
