Underplating generated A- and I-type granitoids of the East Junggar from the lower and the upper oceanic crust with mixing of mafic magma: Insights from integrated zircon U–Pb ages, petrography, geochemistry and Nd–Sr–Hf isotopes

Abstract

Whole rock major and trace element, Nd–Sr and zircon Hf isotopic compositions and secondary-ion mass spectrometry zircon U–Pb ages of eleven granitoid intrusions and dioritic rocks from the East Junggar (NW China) were analyzed in this study. The East Junggar granitoids were emplaced during terminal Early to Late Carboniferous (325–301Ma) following volcanic eruption of the Batamayi Formation. Zircons from the East Junggar granitoids yielded 210 concordant 206Pb/238U ages which are all younger than 334Ma and exhibit εHf(t) values distinctly higher than Devonian arc volcanic-rocks. Seismic P-wave velocities of deep crust of the East Junggar proper resemble those of oceanic crust (OC). These characteristics suggest absence of volcanic rock and volcano-sedimentary rock of Devonian and Early Carboniferous from the source region. The East Junggar granitoids show εNd(t) and initial 87Sr/86Sr values substantially overlapping those of the Armantai ophiolite in the area. The Early Paleozoic OC with seamount-like composition as the Zhaheba–Armantai ophiolites remained in the lower crust and formed main source rock of the East Junggar granitoids. Based on petrography and geochemistry, the East Junggar granitoids are classified into peralkaline A-type in the northern subarea, I-type (I1 and I2 subgroups) mainly in the north and A-type in the south of the southern subarea. The perthitic or argillated core and oligoclasic rim with an argillated boundary of feldspar phenocrysts and inclusion of perthites or its overgrowth by matrix plagioclase, in the monzogranites (northern subarea), suggest mixing of peralkaline granitic magma with mafic magma. In the north of the southern subarea, the presence of magmatic microdioritic enclaves (MMEs) in the I1 subgroup granitoids, transfer of plagioclase phenocrysts and hornblendes between host granodiorite and the MME across the boundary and a prominent resorption surface in the plagioclase phenocrysts indicate mixing of crustal magma (I2 subgroup granitoids) with mafic magma. Magma mixing shifted (87Sr/86Sr)i of the I1 subgroup granitoids towards the mantle array. Two generations of hornblende with zonal distribution and similar mineral and geochemical compositions of quartz monzodiorite and hosted MME with unfractionated rare earth elements (REE) suggest extended magma mixing with onset probably at or near source region. These observations imply concurrency of mantle input and the crustal melting and, hence, a causal relationship between underplating/intraplating and the lower OC/upper OC melting. The I-type granitoids experienced plagioclase and hornblende fractionations, whereas fractionated phases of the two groups of A-type granites were alkali feldspar and albite–oligoclase with significant involvement of F−-rich fluid. Granodioritic parent magmas of the I2 subgroup granitoids stemmed from the hydrous upper OC. Parent magmas of the two A-type groups possess syenogranitic or quartz syenitic compositions. The peralkaline A-type granites stemmed from the lower OC, whereas the A-type granites from dehydrated upper OC left behind after extensive partial melting and extraction of I-type granitoids. Based on comparison in the ternary system Mg2SiO4–CaAl2SiO6–SiO2, most of the Batamayi volcanic rocks with affinity to ocean-island basalts were derived from asthenospheric upwelling. The gabbro–dioritic rocks with higher light to heavy REE ratios stemmed from metasomatized lithospheric mantle. Both of the above mafic rocks contain subducted slab component.Loci of ponding of mafic magma and crustal melting migrated upward and southeastward from the lower OC in the northern subarea (yielding the peralkaline A-type granites) to the upper OC in the southern subarea. The latter evolved from hydrous (yielding the I-type granitoids in the north of the subarea) to dehydrated (yielding the A-type granites in the south of the subarea). Sequence of these events resulted in increasingly-later formation of the East Junggar granitoids southeastwards with an age span of ~20Ma. Minor contribution from Late Paleozoic OC to the source of the southern granitoid subarea caused the latter showing distinctly higher εNd(t) than the northern subarea. In conclusion, underplating caused OC remelting which, together with significant magma mixing, produced the East Junggar granitoids in a non-subduction zone environment.