Chronology And Geochemistry Of Late Hercynian Gabbro And Diorite In Hunchun Area

The Yanbian fold belt is located at the junction of the Jiamusi, Khanka and Longgang massifs, i.e., the easternmost segment of the Hinggan-Mongolia Orogenic Belt (XMOB). The region mainly experienced collision and amalgamation of the Jiamusi, Khanka and Longgang massifs during the Paleozoic. For a long time, the Late Paleozoic tectonic nature between the Khanka and Longgang massifs has always been a controversial issue. At present, two different opinions are presented based on the closure time of the Paleo-Asian Ocean, the late Paleozoic sedimentary strata, and paleontological evidence. One is that the Khanka and Longgang massifs were separated by an aulacogen in the Permian; the other is that the closure of the Paleo-Asian Ocean between the Khanka and Longgang massifs happened in the Permian. In the latter view, some scholars consider that the Paleo-Asian Ocean subducted beneath the southwestern edge of the Khanka Massif. Then, whether did the subduction between the Khanka and Longgang massifs happen or not? How did the Paleo-Asian Ocean crust subduct beneath the Khanka Massif or the Longgang Massif? The above-mentioned problems have been unsolved due to the lack of detailed dating and geochemical data for the magmatic rocks in the region. However, a voluminous Late Hercynian intermediate-basic rocks, occurred in the Hunchun area, provide insight to solve the above-mentioned problems. Therefore, this paper presents zircon U-Pb dating, Hf isotope, and geochemical data for the Late Hercynian gabbros and diorites in Hunchun area to constrain the tectonic evolution between the Khanka and Longgang massifs in the Late Paleozoic.1. Formation time of gabbro and diorite LA-ICP-MS zircon U–Pb dating results show that zircons from the gabbro in Hunchun Area yield a weighted mean 206Pb/238U age of 282±2 Ma, indicating that the gabbro formed in the Early Permian, that the magmatic zircons from the diorite yield a weighted mean 206Pb/238U age of 255±3 Ma, indicating that the diorite formed in the Late Permian, and that the captured zircons from the diorite yield a weighted mean 206Pb/238U age of 279±4 Ma, which is consistent with the formation time of the gabbro under the uncertainty.2. Geochemistry and zircon Hf isotopes of the gabbros and dioritesThe gabbros in Hunchun area have SiO2 = 46.75-47.54%, Mg# = 61-70, high Al and low Ti (Al2O3 = 19.99-20.87%, TiO2 = 0.37-0.41%), belonging to the subalkaline low-K tholeiitic series. Chondrite-normalized rare earth element (REE) patterns indicate that the gabbros are characterized by relatively enrichment in LREEs and depletion in HREEs, and weak positive Eu anomalies. Meanwhile, the gabbros display an enrichment in large ion lithophile elements (LILEs; e.g., Rb, Ba, and K), depletion in high field strength elements (HFSEs; e.g., Nb, Ta, and Ti), and strong positive Sr anomalies, similar chemically to island-arc tholeiites. 176Hf/ 177Hf ratios andεHf(t) of selected zircons from the gabbro vary from 0.282813 to 0.283015 and from +7.63 to +14.61, respectively.The diorites have relatively high SiO2 contents (SiO2 = 54.56-61.23%), lower Mg (Mg# = 37-39), high alkali contents (Na2O+K2O = 4.74-5.19%), belonging to the subalkaline middle-K calc-alkaline series. Compared with the gabbros, the diorites have the similar chondrite-normalized REE patterns and primitive mantle (PM)-normalized trace element patterns, e.g., enrichment in LREEs and large ion lithophile elements, depletion in HREEs and high field strength elements. However, the diorites have obviously higher REE and other trace element abundances, and weak negative Eu anomalies, similar to the igneous rocks from the active continental margin setting. 176Hf/ 177Hf ratios andεHf(t) of zircons with age of 255 Ma from the diorites are between 0.282940 and 0.283040 and between +11.22 and +14.17,respectively. Their Hf model ages range from 424 Ma to 692 Ma.3. Magma sourceThe gabbros in the Hunchun area are poor in SiO2 (46.75-47.54%), rich in MgO (7.56- 8.07%), high Mg# (61-70), and high Cr (201-229 ug/g) contents, suggesting that their primary magma could be derived from partial melting of peridotite mantle.The depletion in Nb, Ta and Ti of the gabbros could exclude the possibility that the gabbro magma could be derived from partial melting of the lithospheric mantle metasomated by the subducted slab-derived melt at the mantle wedge. Therefore, we propose that the primary gabbro magmas should be derived from partial melting of mantle wedge peridotite metasomatized by subduction- related fluid, which is consistent with their high Al and low-K tholeiitic geochemical features and is also supported by their low Zr/Y ratios.εHf(t) values of zircons from the gabbros vary from +7.63 to +14.61, suggesting that primary magma should be derived from partial melting of a depleted lithospheric mantle. Taken together, the gabbro magmas should originate from partial melting of the depleted mantle wedge metasomatized by the subducted slab-derived fluid. Combined with their similarity to volcanic arc low-K tholeiites, it is suggested that the gabbros could form in island arc setting close to trench.The Wudaogou diorites are characterized by enrichment in SiO2 and alkali, as well as depletion of MgO, Cr (3.66 ug/g ~7.29 ug/g) and Ni (1.91 ug/g ~ 5.35 ug/g), implying that they could not be the mantle-derived primary magma. TheεHf(t) values and Hf model ages of zircons from the diorite range from +11.22 to +14.17 and from 424 to 692 Ma, respectively, suggesting that the primary magma for the diorites could be derived from partial melting of the Early Paleozoic and/or Neoproterozoic accreted lower crust.4. Tectonic implicationsThe gabbros in Hunchun area show the geochemical features of the igneous rocks from the island-arc setting, suggestting that the Paleo-Asian oceanic plate subducted beneath the the Khanka Massif in the Early Permian. The diorites, together with the coeval tonalitic rocks from the Xiaoxinancha area, are chemically similar to the igneous rocks from the active continental margin setting, implying that an active continental margin setting could exist in the region in the late Permian. The gabbros and diorites are both located in the western margin of the Khanka Massif, and fromed in the Early and Late Permian, respectively. Spatial and temporal distribution of gabbros and diorites implies that the collision and amalgamation of the arc and continent happened in the region between the Early Permian and Late Permian.