Geochronology and Geochemistry of Late Devonian I- and A-Type Granites from the Xing’an Block,NE China:Implications for Slab Break-off during Subduction of the Hegenshan-Heihe Ocean

We present detailed geochronological,geochemical,and zircon Hf isotopic data for Late Paleozoic granitic rocks from Handagai and Zhonghe plutons in the Xing’an Block,NE China,aiming to provide constraints on their origin and tectonic implications.New zircon U-Pb ages indicate they were formed in the Late Devonian(ca.379 Ma) immediately after a striking 50 Ma magmatic lull(ca.430-380 Ma) in the Xing ’ an Block.Petrological and geochemical features suggest that the Handagai monzogranites and Zhonghe alkali-feldspar granites are I- and A-type granites,respectively,although both of them have high-K calc-alkaline features and positive zircon ε_(Hf)(t) values(+3.47 to +10.77).We infer that the Handagai monzogranites were produced by partial melting of juvenile basaltic crustal materials under a pressure of <8-10 kbar,whereas the Zhonghe alkali-feldspar granites were generated by partial melting of juvenile felsic crustal materials at shallower depths(P ≤ 4 kbar).Our results,together with published regional data,indicate their generation involves a subduction-related extensional setting.Slab break-off of the Hegenshan-Heihe oceanic plate may account for the subduction-related extensional setting,as well as the transformation of arc magmatism from the Early-Middle Devonian lull to the Late Devonian-Early Carboniferous flare-up in the Xing’an Block.

Early Permian A-type Granites in the Zhangdaqi Area, Inner Mongolia, China and Their Tectonic Implications

There is a controversy regarding the amalgamation of Xing’an and Songnen Blocks along the Hegenshan-Heihe Suture(HHS)in the eastern Central Asian Orogenic Belt(CAOB).To solve this problem,we performed detailed study on the granites from the Zhangdaqi area,adjacent to the north of the HHS in the northern part of the Great Xing’an Range,NE China.Geochemically,the granites in the study area are metaluminous-weak peraluminous and high-K calc-alkaline series.Trace elements of the granites show that LREEs are relatively enriched,while HREEs are relatively deficient and obvious REE fractionation.The granites are characterized by obvious negative Eu anomalies,meanwhile,they are relatively enriched in Rb,K,Th and depleted in Ba,Nb,Sr,P,Ti.All the geochemical features suggest that the granites in the Zhangdaqi area are aluminum A-type granites.The zircon LA-ICP-MS U-Pb ages of these granites are 294-298 Ma,indicating that they formed in the Early Permian.These granites also have positiveεHf(t)values(8.4-14.2)and a relatively young two-stage model age between 449 Ma and 977 Ma,implying that the magma was derived from the re-melting of the Early Paleozoic-Neoproterozoic juvenile crust.Combined with geochemical characteristics(Nb/Ta ratios of 9.0-22.2,and Zr/Hf ratios of 52.3-152.0),we believe that the magmatic source area is a mixture of partial melting of the lower crust and depleted mantle.A-type granites and bimodal volcanic rocks along the Hegenshan-Heihe Suture formed during the Late Carboniferous-Early Permian,indicating that the HHS between Xing’an and Songnen Blocks closed in the late EarlyCarboniferous.Subsequently,the Zhangdaqi area was in a post-orogenic extensional environment from Late Carboniferous to Early Permian and resulted in the formation of the A-type granites.