The Era And Petrogenesis Of Raohe Granites In East Heilongjiang Province

There were complete Mesozoic marine sediments distributed inRaohe area, NE Heilongjiang Province, which mainly composed ofdeep oceanic silicalite, clipped mafic to ultramafic complex, pillow lavaand Neopaleozoic neritic deposit and formed a typical tectonic mélange.Lichunyu first taken the conception ophiolite to this area in 1980, andmade this region became a hot research locality for many geologists andthere were many people had made research work on the rocks, stratum,paleobiology, paleomagnetism and tectonic evolution. But there are alsomany problems exist by now, such as (1) the tectonic attribution ofRaohe complex, and (2) the arrival age and arrival methods of Raohecomplex. The problems mentioned above have taken difficulties tounderstand the Mesozoic evolution of the research area. Fortunately, thegranites which have affinity with the complex distributed abroadly inthis area.It gave us a new way to study Raohe complex.Raohe granites mainly distribute in northeast part of Raohe areawith two big outcrops: Hamahe terrene and Taipingcun terrene. Theresearch of the Raohe granites would make good restriction to thearrival age of Raohe complex due to the granites intrude into it.Unfortunately, there were different cognition about the era andpetrogenesis of Raohe granites by now. Heilongjiang Bureau ofGeology and Mineral Resources (1987a, 1987b) gave the whole rockK-Ar age 185-189Ma and 163-175Ma for Hamahe granites andTaipingcun granites respectively. But Shaojián et al(1992) showed thezircon TIMS U-Pb concordant age 115.5±1.5Ma of Hamahe granites,which was more different from the age given above. Although fewpeople do material research on the petrogenesis of the granites in thisarea, there is one trend to tie it in the subduction of the Pacific Plate andthere also has another viewpoint exist which due the reason to thecollision of terrenes and orogenic event followed. In any case, they areall lack of geological data evidence.This text, took the Hamahe terrene and Taipingcun terrene as studyobjects, showed the petrology, mineralogy, geochemistry, chronologyand isotope work on the Raohe granites and expected to present morenew information for the era and petrogenesis of Raohe granites andrestriction to the arrival age of Raohe complex.According to field work and petrology research, Hamahe granitesare mainly composed of granodiorite with the specific types of granuleaplodiorite, medium-fine particle cordierite aplodiorite and porphyritic-like aplodiorite and a little amount of porphyritic-like biotite syeno-granite, porphyritic-like biotite monzonitic granite and plagio-granite.Taipingcun Granites are mainly composed of porphyritic-like graniteswith the specific types of porphyritic-like biotite syenogranite, porphy-ritic-like cordierite 2 mica syenogranite, medium-grain biotite syenogr-anite and a little amount of porphyritic-like biotite monzonitic granite.Geochemical analysis show that both terrenes' rocks contain68%-75% SiO2 and high content of Al, K, Na and low content of Fe,Mg, Ca, Mn. Trace elements data show visible negative abnormality ofBa, Nb, Sr, Eu. The partition model of REE show obvious LREE andHREE fractionation and negative abnormality of Eu element.Both magmatic body rocks appeared cordierite grains in finecrystallization and Aluminous Aaturation Index are more than 1.04,which are very similar to the S-type granite in Lachlan Fold Belt inAustralian after have compared with. Therefore we think Raohe granitesare the S-type granites in genetic type.The zircon U-Pb isotopic age data of Hamahe granites andTaipingcun granites show that both magmatic bodies are formed inCretaceous with Hamahe granites mainly formed at ab.124Ma,probablyat former 131Ma and later 116Ma there also have magmatism whichhave relative with the formation of Hamahe Body, and Taipingcungranites mainly formed at ab.111-114 Ma.The zircon U-Pb age of the gabbro in Raohe complex is 166±1 Ma.According to the Radiolarian in the silicalite, the abyssal sediments areformed at Triassic to Early-Middle Jurassic. That means the gabbroformed later than the deep oceanic sediments, which indicates thatmafic-ultramafic rocks and pillow lava in Raohe complex are intrusivemass into the oceanic crust, after which has received the sediments butthe oceanic crust which formed in mid-oceanic ridge.The era of latest Radiolarian in deep oceanic sediments is 160-150Ma, combine which and the U-Pb age of Raohe granites we canconclude that the emplacement of Raohe complex took place atab.150-131Ma of Later Jurassic-Early Cretaceous. The paleomagneticproof show that Raohe terrene had moved toward north from the lowlatitude area to high latitude area in Later Triassic-Jurassic, indicatingthat there existed the Pacific Plate subduction and striking-slip in thisarea before and during Late Jurassic-Early Cretaceous.Hf isotopic analyses show that the zircons from plutons have high176Hf/177Hf ratios and positive εHf(t) values, and the Hf isotope modelage with the felsic crust rocks as the source is 500-780 Ma, whichindicates that their protolith should be the juvenile crust formed in NeoProterozoic-Phanerozoic. As S-type granites usually formed by fractionmelting of sediments in the Earth, which indicates that there may beexist sediments under the complex. Therefore, it is suggested that thereexisted significant crust uplift and weathering before Late Jurassic-Early Cretaceous in NE China and Raohe area, as mass receive area,had received a large amount of weathering products.