Mesozoic tectonic nature of Southwest Borneo:Banda Arc vs.Indochina?
The Mesozoic Paleo-Pacific tectonic evolution in Southeast(SE)Asia is a research focus in domestic and foreign researchers.Previous studies mainly focused on the igneous and sedimentary rocks along the southeast coastal areas of China and Indochina Peninsula.However,West Borneo of Central Indonesia,the southern extension of the Paleo-Pacific domain,lacks detailed studies on the geochronology,geochemistry,and tectonic nature,which hamper our full understanding on the tectonic pattern of Mesozoic Paleo-Pacific Ocean in SE Asia.The Schwaner Complex in Southwest(SW)Borneo,the largest batholith in Borneo,has preserved abundant Mesozoic magmatic and sedimentary records,and is a key area for investigating the Mesozoic subduction of Paleo-Pacific and tectonic nature of SW Borneo.However,previous studies have two different views of Banda Arc and Indochina origin on the Mesozoic tectonic nature of SW Borneo.To solve the issues above,we have carried out a detailed geological survey and newly identified a series of Cretaceous intermediate-mafic igneous rocks from the Kerabai Formation in the Schwaner Complex.In this study,we present new zircon U-Pb geochronology and Lu-Hf isotopes,as well as systematic whole-rock elemental and Sr-Nd-Pb isotopic data for these newly identified igneous rocks to constrain their formation age,geochemical characteristic,petrogenesis,and tectonic setting.The Cretaceous intermediate-mafic igneous rocks were collected along the Ketapang-Palangkaraya area and consist of basalt,basaltic andesite,andesite,and diabase.Zircon geochronology shows that these Late Cretaceous intermediate-mafic igneous rocks were formed at the Early Cretaceous period(101-90 Ma).These intermediate-mafic igneous rocks are characterized by SiO2(49.39wt％-60.15wt％),TiO2(0.63wt％-1.75wt％),A12O3(14.87wt％-18.95wt％),and Fe2O3t(4.1 lwt％-14.70wt％)with Mg# values of 28 to 59.All these igneous rocks belong to the calc-alkaline to high-K calc-alkaline series.The intermediate volcanic rocks with high MgO contents can be classified as high-Mg volcanic rocks.All these igneous are enriched in LREEs and LILEs and depleted in HFSEs(Nb,Ta,and Ti)with(La/Yb)N and(Gd/Yb)N ratios of 3.27-13.22 and 1.25-2.32,respectively.Their whole-rock initial(87Sr/86Sr)i ratios range from 0.70450 to 0.70635,εNd(t)values from-1.6 to+1.7,and zircon in-situ εHf(t)values from-0.9 to+12.3.They have(206Pb/204Pb)i ratios of 18.73 to 18.83,(207Pb/204Pb)i ratios of 15.66 to 15.67,and(208Pb/204Pb)1 ratios of 38.78 to 39.88,and show Pacific MORB-like Pb isotopic signatures.Geochemical signatures suggest that these intermediate-mafic igneous rocks were derived from the metasomatized mantle wedge modified by slab-derived components,and were formed in a continental-arc setting.Regional comparisons suggest that the Jurassic to Cretaceous arc-like igneous rocks in the SW and NW Borneo have similar Sr-Nd-Pb-Hf isotopic compositions with positive to near to zero εNd(t)values,Pacific-like Pb isotopic values,and positive in-situ εHf(t)values.Besides,four consistent magmatism periods can be identified from the SW and Northwest(NW)Borneo including～85,～110,～135,and～155 Ma.These data consistently indicate that the SW Borneo and NW Borneo were located at the southeastern margin of Sundaland during the Early Mesozoic,and there is a lack of an Early Mesozoic suture boundary between them.These data further confirm that the SW Borneo is not originated from the Banda Arc.SW Borneo and NW Borneo were jointly controlled by the westward subduction and slab-rollback of Paleo-Pacific Ocean during the Late Cretaceous.
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