Subduction And Cu-Au Mineralization: Comparing Studies Of The Early Cretaceous Volcanic Rocks And Adakitic Rocks In Luzong Basin, LYRB And Central Philippines

In this thesis, we mainly focused on the Mesozoic magmatism and mineralization related to the subduction of paleo-Pacific Plate. There are two main parts:associations of shoshonitic volcanic-intrusive rocks and adakitic series in the Luzong volcanic basin and Shaxi Cu-Au deposits, respectively; and the associations of early Cretaceous arc volcanic rocks and Cu-Au mineralizationin related adakitic diorites. According to the combining study of their geochronology, petrology and geochemistry, we attemptted to reveal their genesis and the relations to the Cu-Au mineralization. We tried to constrain their tectonic background by considering the rock series and the spatial and temporal distributions of similar magmatic rocks in adjacent regions. We also tried to discuss the oxygen fugacities of Cu-Au related diorites and Qtz-diorites in Cebu Island and Luzong area by comparing them with other adakitic rocks at the LYRB and the Dabie orogen. The followings ate the details. 1). Associations of shoshonitic rocks and adakitic rocks in the Luzong volcanic basinShoshonitic volcanic rocks are related to large iron deposits, and sodic adakitic plutons in Shaxi district are related to porphyry copper-gold deposit. The Qtz-diorite (high silica adakite, HSA) and newly found high Sr/Y ratio Bt-gabbros and Bt-diorites are with emplacement ages of ca.130-129 Ma, later than that of the early stages Luzong shoshonitic rocks (ca.134-130 Ma). They have similar characteristics of major elements (SiO2, Al2O3, Fe2O3 contents) and show similar distribution patterns of trace elements and REEs:enriched in LILEs and depleted in HFSEs, enriched in LREEs and depleted in HREEs, flat HREE patterns and no Eu negative anomaly for most of these rocks except shoshonitic intrusitions and Fushan Fm. volcanic rocks in the last stage.The εHf(t) values of the shoshonitic rocks in Luzong basin are mostly negative, indicating involvement of crustal materials, their Sr-Nd-Pb isotopic compositions futher indicate MORB and subducted sediments in their sources. The high silica adakites exhibit positive εHf(t) values, which may be attributed to the contribution of subcucted oceanic crust, their Sr-Nd-Pb isotopic compositions also exhibit characteristic of MORB and subducted sediments in their sources, distinct from previously reported lower continental crust (LCC) derived adakites/granites from the Yangtze Craton. Besides, the shoshonitic rocks show more subducted sediments affinities than that of adakitic rocks. They are also distinct from each other in some other aspects:shoshonitic rocks have high K2O+Na2O contents, high K2O/Na2O ratios and high Nb contents, variable Mg#, low Ce/Pb and Sr/La, Low Sr/Nd, high Th contents, variable Th/Yb, suggesting that they were from partial melting of an enriched subcontinental lithosphere mantle (SCLM) which is metasomatised by the fluids/melts generated by a sediments dominated source. Shaxi high silica adakites have high MgO contents, Mg#, Sr contents and Sr/Y ratios, low (La/Yb)N, decoupling Sr/Y and (La/Yb)N, exhibiting characteristics of slab melt. The features of Th/Yb-Sr/Nd, Th/U-Th further support the fact that they are derived from partial melting of subducted oceanic crust with minor overlying sediments, coincident with the conclusion of Sr-Nd-Pb and Lu-Hf isotopic results. The high Sr/Y Bt-gabbros and Bt-diorites are generated by partial melting of a composite subcontinental lithosphere mantle (SCLM) which is metasomatised by both of the dehydration fluids and adakitic melt. The associations of adakitic rocks and shoshonitic rocks support the oblique ridge subduction of the paleo-Pacific plate along the Lower Yangtze River Belt in Mesozoic, and it is probably the cause of large scale of Cu-Au mineralization there. 2). Associations of arc volcanic rocks and adakitic rocks in the Cebu island, Cenral Philippines.The Philippine island arc system is a collage of amalgamated terranes of oceanic, continental and island arc affinities. Here we investigate a volcanic suite in the Cebu Island of central Philippines, including basalt, diabase dike, basaltic pyroclastic rock and porphyritic andesite. LA-ICP-MS U-Pb geochronology of zircon grains from the porphyritic andesite, pyroclastic rock and diorite yielded ages of 126.2±2.4 Ma,118.5 ±1.2 Ma and 112.5±1.5 Ma, respectively, indicating an Early Cretaceous age. The age distribution of the detrital zircons from river sand in the area displays only one peak at ca.118 Ma, close to the age of the pyroclastic rock. The early Cretaceous volcanic rocks in the central Philippines were previously regarded as parts of ophiolite complexes by most investigators, whereas the Cebu volcanics are distinct from these, and display calc-alkaline affinity and island arc setting, characterized by high LREE/HREE ratios and low HFSE contents. These features are similar to the Early Cretaceous arc basalts in the Amami Plateau and east Halmahera in the northernmost and southernmost West Philippine Basin respectively. Zircon Hf isotopes of the pyroclastic rocks show depleted nature similar to those of the Amami Plateau basalts, implying the subducted Pacific-type MORB as probable source. Zircon Hf isotopes of the porphyritic andesite show slight enrichment relative to that of the pyroclastic rocks and MORB, indicating subducted sediments as a minor end-member in the source. The Hf isotopic compositions of the volcanic rocks are also reflected in the detrital zircons from the river sands, with two pronounced peaks in the TDMI ages.The study area is located adjacent to the Atlas Cu-Au deposit, and there is also Cu mineralization observed at the contact zone between the diorite intrusion and the intruded pyroclastic rocks. The diorites in the study area also share closed forming ages and similar geochemical characteristics with the ore bearing diorites in the Atlas mining, indicating that they are probably from one large intrusions with the same source. They are characterized by LREE enrichment and HREE depletion, along with other adakitic sigantures such as high Sr/Y, La/Yb ratios and low Y and Yb contents. They are also classified as high magnesian adakites by high MgO contents and Mg#. Besides, their characteristics of low K2O contents and K2O/Na2O ratios, and decoupling of Sr/Y and La/Yb ratios are similar to those of adakitic rocks in the LYRB and other arc settings, indicating partial melting of subducted oceanic crust. The low Nb, Ta contents and low Nb/Ta ratios are due to partial melting of amphiolite to Amp-eclogite facies. Their Eu/Eu* values and medium to high Ce4+/Ce3+ratios indicate that the oxygen fugacities of the magma are high, thus favor for Cu-Au mineralization.We propose that the volcanic rocks of Cebu Island were derived from partial melting of sub-arc mantle wedge which was metasomatised by dehydration of subducted oceanic crust together with minor pelagic sediments, while the adakitic rocks are derived form the partial melting of subuducted oceanic crust. The combination of normal arc volcanic rocks and adakitic rocks in the area are related to the subduction and slab roll back process of the old oceanic crust. Within the tectonic environment of Southeast Asia during Early Cretaceous, the volcanic rocks in Cebu Island can be correlated to the subduction and slab roll back processes of paleo-Pacific plate. The Early Cretaceous volcanic suites in the Cebu Island along with the Early Cretaceous arc volcanic rocks, ophiolites and boninites in the southern-western-northern margin of West Philippine Basin are inferred to have formed within single subduction zone before the opening of the Philippine Sea Plate.