Magma Activities At Convergent Margin

Convergent plate edges being plate destruction generally include subduction tectonic environment and collision tectonic environment. It can be divided into the ocean-continent convergence and continent-continent convergence, as an important kind of plate tectonic environment. On the edge of the convergent plate, the plate subduction can carry amounts of the surface materials into the deep mantle for materials recycling or elements differentiation. These processes play an important role in continental crustal growth, magmatic activity and metal mineralization. At the same time, late collision orogeny also makes a significant contribution to continental crust deformation, metamorphism and magmatic activity. Thus, studying the convergent plate edge magmatic activities is able to better understand the earth material migration, recycling and evolution. In this thesis, we focus on the Nagqu Mesozoic magmatic rocks in the northern Lhasa subterrane, Tibetan Plateau and Xu-Huai region Liguo intrusive complexes in the southeast margins of North China Craton to study the convergent plate edge magmatic activity features.Since Paleozoic, the Tibetan Plateau, as the world’s typical subduction collision orogenic belt, has experienced the Tethys Ocean lithosphere subduction and closure between the Laurisia land and the Gondwana land, and then the India-Eurasia continental collision. The Lhasa terrane, as the southern-most margin of the Asian continent, records a large number of information about the subduction collision orogeny, which is one of the important parts to study the convergent plate edge’s magmatic activity in Tibetan Plateau. Though the predecessors had much research on Lhasa terrane, there still remain many controversial issues, such as the origin, basement, tectonic evolution, petrogenesis and deep dynamics, etc. In addition, Mesozoic magmatic rocks in the northern of the Lhasa terrane have positive εHf(t) value similar to the southern of the Lhasa terrane, suggests that the continental crust growth in the Phanerozoic. However, there is still unclear for the process of the continental crust growth and the deep dynamic process, due to the scarcity of petrological, geochemical, geophysical evidence. In this study, we focus on the late early Cretaceous intrusive rocks and volcanic rocks of Nagqu area, with the aid of the field geological investigation, mineralogy, petrology, isotope geochemistry(Hf-O) and chronology(zircon U-Pb), to reveal the petrogensis and tectonic environment of ~110 Ma magma explosion incident, and even limit the origin or basement of the Lhasa terrane.Intrusive rocks in the Nagqu area were formed at early Late Cretaceous, which located in the northern of the Lhasa terrane, consisted of biotite granites and biotite monzogranites. LA-ICP-MS zircon U-Pb dating of biotite granites and biotite monzogranites in Nagqu area yields magmatic crystallization ages of ca. 112 Ma, which indicate that they were emplaced in the late Early Cretaceous. Both rocks show similar geochemical characteristics, and have high-K calc-alkaline to shoshonitic composition and slightly- moderately peraluminous signature. They are enriched in the alkalis, Rb, Th, K, U and light rare earth elements, depleted in Nb, Ta, Ti, P, and characterized by high Al2O3 contents(12-16 wt.%), high Rb/Sr ratios(1.3-33) and low Mg# values(15-39). Their magmatic zircons have negative εHf(t) values(from-25.9 to 0.5) and high positive δ18O values(from 7.9‰ to 11.5‰). All the above characteristics indicate that Nagqu intrusive rocks were likely derived from hybrid melts of sediments from the continent crust with minor mantle-derived input, and then experienced varied degrees of fractional crystallization. The Nagqu intrusion is a component of the late Early Cretaceous magmatic flare-up event that occurred during ~120-100 Ma in the northern and partly central Lhasa subterranes. This magmatic flare-up is marked with a large compositional diversity(basalt, rhyolite, adakitic rocks, dioritic enclave, biotite monzogranite and biotite granite), previous studies considered that it may be caused by the slab breakoff of the southward subduction Bangong-Nujiang oceanic lithosphere. We conclude that it may be associated with Neo-Tethys ocean closure, especially Neo-Tethys ridge subduction, causing slab window opening and then asthenosphere upwelling, which may have significantly contributed to juvenile crustal growth of the northern Lhasa subterrane.The volcanic rocks in the Nagqu area consist of andesite, rhyodacite and rhyolite. In this study, trachyandesites were formed in 111.9±1.2 Ma while dacites formed in 109.2±3.5 Ma based on LA-ICP-MS zircon U-Pb dating, belong to the products of(~112 Ma) magmatic flare-up event in the northern and central of the Lhasa subterranes. These rocks show a slightly peraluminous signature with variable Si O2(51.88 wt.%~74.62 wt.%), low Mg O(0.37 wt.%~3.65 wt.%) and low Mg#(16~40). They also display enrichment in Th, U, LREE and depletion in Nb, Ta, P, Ti, HREE with obvious LREE and HREE fractionation((La/Yb)N=8.8~14.9) and negative Eu anomalies(δEu=0.46). Temperature calculated by Ti-in-zircon thermometer is ca.720℃ and the Ce(Ⅳ)/Ce(Ⅲ) of zircon(2-250) are generally less than 200, suggesting a magma source with low temperature and low oxygen fugacity, far away the subduction zone. Zircons have the variable negative εHf(t) values(from-7.7 to 0.5), corresponding to crustal Hf model ages of 918~1336 Ma. Zircons have positive δ18O values(from 4.57‰ to 9.46‰), are higher than that of mantle zircons, close to the sendimentary zircons, suggesting the contribution of continental crust materials. In addition, andesite zircon cores(106 Ma and 108 Ma) have more negative εHf(t) values(-16.6 and-30.4) and older crustal Hf model ages(1785 Ma and 2481 Ma) than other spots, positive δ18O values(5.10‰ and 7.06‰), indicated that the residual ancient continental materials is also involved in the formation of Nagqu volcanic rocks. Combining with petrology and geochemical characteristics, it has been suggested a mixing source from lower crust and depleted mantle for Nagqu volcanic rocks. They can be considered as the products of partial melting of crust-mantle mixing, which may be connected with Neo-Tethys ocean closure, especially Neo-Tethys ridge subduction causing slab window opening, or flat subduction, plate rollback, and then the melts have experienced different degrees of fractional crystallization.Though the early tectonic evolution of the Lhasa terrane, central Tibetan Plateau, remains poorly understood, more and more old materials have been reported recently. Fortunately, we have analysed in-situ U-Pb dating and Hf-O isotope of diorites. Age of the diorite is ~111 Ma. We get a surprising package of inherited zircons. Importantly, the oldest zircons are euhedral. The discovery of Paleoarchean zircons in the region suggests potentially old rocks. Here we report the results of nearly 300 zircons from Nagqu quartz diorite. The zircons yield five age peaks at ~3.45 Ga, ~2.56 Ga, ~1.76 Ga, ~900 Ma and ~111 Ma. These zircons have large variable εHf(t) values(from-45.1 to 9.2) and old Hf model ages(from 924 Ma to 3935 Ma), and variable δ18O values of-5.80 to 9.64. Zircon age populations of ~2.56 Ga, ~1.76 Ga and ~900 Ma suggest that the Lhasa terrane experienced crustal reworking and growth during Neoarchean, witnessed the Late Paleoproterozoic tectonic magmatic event, and had an affinity with Qiangtang, Himalaya and Indian block. The inherited oldest zircon grains have Paleoarchean ages(~3.5 Ga), one of them has negative εHf(t) value(-4.8), old Hf model age(3935 Ma) and high positive δ18O value(7.27) indicating the existence of the ancient continental crust. Remarkably, these zircon grains are euhedral, indicating that they either are proximal, or the zircons were present in lithic clasts that have been broken down and released into the quartz diorite either by assimilation or by digesting deep down. In both cases, the old zircons indicate that there is Paleoarchean basement near Nagqu in the Tibetan Plateau.The Archean North China Craton(NCC) attracts attention due to the large scale craton destruction(lithosphere thinning) in the Mesozoic era. Although much research results have been made, there are still in debate on destruction mechanism of thinning and deep dynamics process of the Mesozoic eastern NCC. Adakites are the key point to solve the above problems. Therefore, based on the basis of predecessors’ materials, with the aid of petrology, isotope chronology and isotope geochemical analyses of Liguo intrusive rocks in the Xu-Huai region, reveal the petrogenesis, the mystery of Pb isotope of the adakites and the mechanism of the NCC destruction.Liguo intrusive rocks in the Xu-Huai region consist of granodiorites and hornblende diorites, were formed in early Cretaceous, and also were associated with Cu-Fe mineralization. Liguo intrusive rocks have high Si O2(64.0-71.6 wt.%)、K2O(2.95-3.70 wt.%), Na2O(4.78-5.52 wt.%), and K2O/Na2 O ratios(0.54-0.75), with the characteristics of enrichment in Na2 O. They also display enrichment in K, Rb, Ba, Sr, LREE and depletion in Nb, Ta, Th, Zr, HREE with obvious LREE and HREE fractionation((La/Yb)N=12.6~26.7) and no obvious Eu anomalies(δEu=0.46). Liguo intrusive rocks have high Sr(502-655 ppm), low Y(3.0-9.1 ppm) and Yb(0.25-0.98 ppm), with high Sr/Y ratios(55.9-178.3), having the typical characteristics of adakites. According to high Mg# and low Fe OT/Mg O values, Liguo intrusive rocks were classified as high Mg adakitic rocks which were derived from partial melting of the delaminated(eclogite-faces) lower crust or plate subduction model. Previous studies showed that the lower crust inclusions included in adakites are dry, different from high water magmas, thus they have different source. Moreover, previous reports of enrichment of Nb basalts and xenoliths of amphibolite faces retrograde metamorphism also can`t explain using delamination model. The samples Sr-Nd isotope(87Sr/86Sri=0.70615-0.70670, εNd(t)=-3.72~-7.13) are similar to subduction ocean sediments. The high radiogenic Pb isotope data((206Pb/204Pb)t=18.041-18.990,(207Pb/204Pb)t=15.531-15.637,(208Pb/204Pb)t=24.854-38.848) are similar to the Yangtze basement data, indicating the contribution of Yangtza basement. However, our samples defined a perfect Pb-Pb isochron(~1.95 Ga) with previous reported data, are close to Wuhe group metamorphic rocks, implying large-scale Pb isotopic homogenization of the Xu-Huai region by the early Proterozoic granulite faces metamorphism. The inherited zircons of Liguo adakitic rocks yield ages of ~1.8 Ga, ~2.5 Ga, corresponding to age of NCC basement. On adakitic discriminant figures, these rocks belong to the partial melting of subducted oceanic area. Overall, the above evidence supports(~130 Ma) ridge subduction is the main mechanism of NCC destruction. The mechanical erosion of low angle subduction, and then the thermal erosion of adakitic magma and A-type granite magma play a key role in NCC destruction.