Petrogenesis and Geological Implications of Late Paleozoic Intermediate-Basic Dyke Swarms in Western Junggar
He Xinxing
Xiao Long
Wang Guocan
Gao Rui
Yang Gang
Yan Shengwu
Abstract：Understanding of the formation and occurrence of intermediate-basic dyke swarms can facilitate future studies on the magmatic evolution and tectonic settings.The Xiaerpu and Xiaoxihu intermediate-basic dyke swarms in West Junggar are prin-cipally composed of dioritic porphyrite,with minor diabase.LA-ICP-MS zircon U-Pb dating results indicate that the Xiaerpu dyke swarm is emplaced with age of 308.6 ±5.5 Ma.It is concluded that these dykes are the products of magmatism in Late Carboniferous based on the geological relationship between the dykes and host granitic rocks,and previous chronological stud-ies.The dykes are characterized by high Mg# (>40),high MgO(>3%)and Al2 O 3 (>1 6%),belonging to calc-alkaline rocks and metaluminous rocks (with A/CNK approximately at 0.9 and A/NK >2).The rocks have lower total REE (most are con-centrated in 40×10 -6 -60 ×10 -6 ),with enriched LREE and slightly positive Eu anomalies ((La/Yb)N =3.03-1 1.32,δEu=1.00-1.20).All samples are highly depleted in (Nb,Ta,Ti and Th),and enriched in (Sr,Ba,K and Rb).Meanwhile,the high contents of Sr (>500×10 -6 ),low contents of Y and Yb,high values of Sr/Y ratios (36.98-95.74),indicate the rocks (except sample XEP01,XEP02 and XXH06)are analogous to Mg-enriched adakitic rocks.Dykes have depleted Sr-Nd isotopic composition ((87 Sr/86 Sr)i =0.703 58-0.703 80,εNd (t )=5.76-6.34).Geochemical and isotopic compositions indicate that de-pleted-mantle components and materials related to subduction have prominent significance in the origin source of these dykes. Combined with the district geological evolution history and previous studies,we propose that West Junggar was under a post-collisional regime in Late Carboniferous.Due to the melting of remnant subducted oceanic slab,the slab melts interacted with the mantle materials to varying extent during ascent,and then accompanied fractional crystallization of clinopyroxene that gen-erated the Mg-enriched adakitic rocks in Xiaerpu and Xiaoxihu intermediate-basic dyke swarms;and fractional crystallization of clinopyroxene of depleted-mantle materials metasomatised by slab melts can account for the formation of the diabase in Xiaerpu dyke swarm and the amphibolic diorite in Xiaoxihu dyke swarm.The emplacement of large-scale intermediate-basic dyke swarms further show that West Junggar was under a post-collisional extensional environment in Late Carboniferous.
Keywords：western Junggarintermediate-basic dyke swarmremnant subducted oceanic slabdepleted-mantlepetrologygeochemistry
Publication Date：2015-01-01
Online Publishing Date：2025-08-15(First online date of this platform, not the publication date of the document)
Pages：20( 777-796 )