Abstract:What are the Paleo- Pacific subduction- related geological signatures preserved in Borneo? Whether are the NW and SW Borneo fragments separated by an unknown suture? Whether do the Mesozoic igneous rocks in West Borneo resemble those in South China Coastal Province? Answering these queries are the keys for better understanding the Mesozoic Paleo- Pacific arc- trench system in East Asia continental margin. This paper synthetically presented the field observations and whole- rock elemental and Sr- Nd- Pb isotopic compositions, along with zircon U- Pb geochronological and Hf- O isotopic data for the Mesozoic igneous rocks from NW and SW Borneo. These data reveal the development of Triassic (ca. 256~216 Ma) granitoids in NW Borneo with the derivation of the metamorphic basement with the input of the juvenile crustal materials. In the Sarawak Kuching zone, the Pakong and Sarabang mafic rocks in the Lupar Line, also Lupar ophibolite mélange, the previously- mapped Triassic Serian volcanics and the Sematan, Lundu and Sri Aman granitoids are dated at ca. 98~84 Ma, ca. 95~77 Ma and 82~77 Ma, respectively, also synchronous with the deposition time of the Pedawan sedimentary sequence. The Late Cretaceous Pakong- Serabang mafic rocks are characterized by high MORB- like εNd(t) values. The Late Cretaceous Serian volcanics and associated granitoids in the Sarawak Kuching zone show arc- like elemental and isotopic signatures. Such data suggest a Cretaceous arc- trench system along the Lupar Line. In NW Kalimantan of Indonesia, the Bengkayang Formation, previously mapped as Upper Triassic- Lower Jurassic sedimentary sequence, is constrained to be Late Jurassic in age. The Raya volcanics was dated at ca. 144~130 Ma. In addition, this study also reveals development of Late Jurassic (~155 Ma) Bengkayang granitoids and Early Cretaceous (ca. 140~130 Ma) Mensibau batholith in NW Kalimantan. In SW Borneo, the Kuayan and Ketapan meta- sedimentary sequences are re- constrained at Upper Triassic- Lower Jurassic or Lower Jurassic rather than previously- considered Upper Paleozoic sequences at which the sedimentary debris were from the Indochina- East Malaya active continental margin. The Pinoh metamorphic Complex and associated Menunuk volcanics, originally- mapped Paleozoic metamorphic basement, were dated at Early Cretaceous (mainly ca. 135~120 Ma). In the previous- mapped Kerabai volcanics, three U- Pb age- groups of ~190 Ma, ~155 Ma and ~90 Ma are recently identified, re- defined aso the early Jurassic (~190 Ma) Betenung, Late Jurassic (~155 Ma) Kudamgan and Lare Cretaceous (ca. 102~85 Ma) Kerabai volcanic sequences. New data indicated that the Sukadana batholith in SW Borneo might be constituted by the Early Jurassic (~190 Ma) Belaban, Late Jurassic (ca. 160~150 Ma) Mentembah and Late Cretaceous (ca. 87~72 Ma) Sukadana batholith, rather than previously- defined Late Cretaceous batholith. All data collectively suggest the development of four igneous events of ca. 200~190 Ma, ca. 155 Ma, ca. 140~125 Ma and ca. 99~77 Ma) in NW and SW Bormeo, respectively. These events spatially show a propagating trend from west to east, resembling those in the Southeast China Coastal Provinces. Whether in NW Borneo or SW Borneo, the Early- Late Jurassic and Early Cretaceous intermediate- mafic and felsic igneous rocks have similar Sr- Nd- Pb- Hf- O isotopic compositions, positive or near to zero εNd(t) values, Pacific- like Pb isotopic signatures, and positive zircon in- situ εHf(t) and mantle- like δ18O values，suggesting the derivation of the subduction- related mantel wedge or juvenile mafic crust, respectively. These data consistently indicate that the NW and SW Borneo were located at the southeast margin of the Indochina- East Malay fragment of the Sundaland old- land at Early Mesozoic. An easterly- propagating Andean- type accretionary orogenesis occurred in West Borneo during the Earliest Jurassic (likely Triassic)- Late Cretaceous period, with the suture boundary along the Lupar Line and Palangkaraya of Central Kalimantan at Late Cretaceous. As a result, a long- lived (>120 Ma) Paleo- Pacific Andean- type accretionary orogen has been created along the East Asian continental margin, from West Borneo, northerly into Southeast China Coastal Province and SW Japan across SE Vietnam and northern South China Sea. Such an orogenesis initiated at the Earliest Jurassic (~200 Ma) or earlier, and ended at the Latest Cretaceous and was controlled by the “multiple subduction and rollback” of Paleo- Pacific Ocean.
