The Luojiashan gabbro is a newly discovered mafic pluton emplaced in the Neoproterozoic Yingyangguan Formation in Yingyangguan region of northeastern Guangxi,South China.Comprehensive whole-rock geochemical and zircon geochronological and Hf isotopic analyses are performed on the gabbro and comparisons are made with the coeval mafic-ultramafic sills and dikes located in Longsheng region of northern Guangxi in order to understand the magmatic origin,evolution,and tectonic setting of the Luojiashan gabbro and to address the location of the suture zone of the Southwestern Jiangnan Orogen(SJO).LA-ICP-MS zircon U-Pb ages obtained from the Luojiashan gabbro show that it was emplaced at~770 Ma.The gabbro contains abundant inherited zircons aged at 0.9-1.3 Ga,consistent with age spectrum of the Cathaysia Block.Chondrite-normalized REE pattern,primitive mantle-normalized trace element spider diagram,incompatible element ratios of Nb/Ta,Zr/Hf,La/Nb,Ba/Th,Th/La,and Ba/La,and Th/Yb-Ta/Yb discrimination diagram of the gabbro are indicative of OIB-like geochemical characteristics and of derivation from partially melted garnet peridotite of the asthenospheric mantle.Tectonic discrimination based on the trace and rare earth elements also indicate that the Luojiashan gabbro was emplaced in a within-plate extensional rift setting,probably as a result of Rodinia supercontinent dismantling,lithospheric thinning,and underplating and upwelling of the asthenospheric mantle.Based on zircon age,Hf isotopic data and comparison between northern and northeastern Guangxi,it is suggested that the Yingyangguan region was tectonically situated in a different tectonic locale from the Longsheng region of northern Guangxi at about 770 Ma during the post-orogenic mafic-ultramafic magmatic event,with the former within the Cathaysia Block and the latter along the southeast margin of Yangtze Block.The suture between the Yangtze and Cathaysia blocks must be located between Yingyangguan of northeastern Guangxi and Longsheng of northern Guangxi.
Ya QinZuohai FengJiaming ZhuYonggao HuangJie WuYun ZhouYunfeng XueChunzeng Wang
Granitoids at Butre area, south-western part of the Birimian of Ghana are gabbro to granodiorite which intruded comagmatic basalt and andesite of volcanic island arc setting. Differentiation was from probably mantle fractionates on tholeiitic basalt trend through gabbro-diorite and granodiorite. Yet, the diorite intruded into the volcanic rocks prior to plate collision. Plagioclase feldspars of labradorite to andesine compositions (An52 - An48), amphibole and pyroxene are in association with accessory tourmaline, actinolite, apatite and garnet. Relict amphibolite facies metamorphism is preserved in xenoliths in gabbro to diorite which show greenschist facies. Generally, plagioclase is partially altered to fine quartz, sericite and carbonates whereas chlorite and epidote are of amphibole alteration. These alterations are typical of carbonatisation but diorite shows partial carbonate-sericite alteration. Alterations followed granodiorite emplacement. Arsenopyrite and pyrite accompanied emplacement of diorite and granodiorite respectively while amphibolite facies metamorphism introduced magnetite;haematite and pyrrhotite accompanied greenschist facies metamorphism. Earlier opaque minerals possibly were consumed to form later minerals. All the rocks are LREE depleted (La/Sm chondrite normalised ratios of basalt = 0.8 to 2.5;andesite = 2.03;diorite = 1.95 to 3.1;gabbro-diorite = 0.9 to 2.05;granodiorite = 1.66);diorite, basalt and andesite are fairly enriched in HREE. Enrichment of FeOT, MgO, SiO2, CaO and depletion of TiO2, K2O and Zr might be linked with carbonitisation, sericitisation, chloritisation, silicification and sulphidation linked to gold mineralisations in the Birimian of Ghana.
