Volcanic Rocks In The Central Belt Of Tethyan Himalayas Since The Late Paleozoic And Their Implications

Studies of Tethyan Himalayas offer the opportunity to reveal the important geologicalprocesses that are related to the continental breakup of northern margin of Gondwana, platedrifting, continent converging and the formation, development, closure of Tethyan ocean basin.More attentions have been focused on this area for a long time. However, the previous studies aremainly focused on the sedimentology, paleontology and tectonics/structural geology to infer theimportant geological processes that mentioned above. There is the fact that a poorly known for thevolcanic rocks in the Tethyan Himalayas, these would be ascribed to the inadequate geologicalinvestigation and the neglect of importance of the volcanic rocks in Tethyan Himalayas. As acarder of information for the interaction of crust-mantle, lithosphere-asthenosphere, petroprobeand indicator for most important geological processes, volcanic activity has been playing animportant role in the study of petrology and tectonics. Fortunately, here is a possibility to study thecontinental breakup of northern margin of Gondwana and the tectonic evolution of Tethys inferredfrom the volcanic rocks, which have been found in the process of our fieldworks and othergeological investigations.As viewed from the main idea of regional tectonic evolution-volcanic activity event, theprimary purposes of the present thesis are to study the features, spatial-temporal evolution,petrogenesis of the volcanic rocks in Tethyan Himalayas since the Late Paleozoic, and to probeinto their implications in the process of continental breakup of northern margin of Gondwana andthe tectonic evolution of Tethys. Some preliminary results have been inferred from these volcanicrocks by the present thesis as following.(1)Based on the re-appraisement of the studies by previous workers and recent researchers,the present thesis points out that the tectonic evolution of Tethyan Himalayas is related toformation and development of the Yarlung Zangbo back-arc basin which is constrained by thesubduction southward of Tethyan oceanic crust that is reconstituted by Bangong Co-Nujiangsuture zone.(2)11 volcanic rocks horizons-bearing, which are characterized by lens, thinned interlayersand massive basalts, have been found in the central belt of Tethyan Himalayas from Permian toCretaceous, the fact indicates that there are frequent volcanic activities with limited scales sincethe Late Paleozoic. From the Permian→the Early/Middle Triassic→the Late Triassic→the Jurassicto the Cretaceous, the volcanic activities move from west to east, and from south to north, to southand to north again.(3)The geochemical and evolutional characteristics of these volcanic rocks have been firstreported in the present thesis. According to the detailed results of geochemical studies, whether thecontent of major elements, REE, trace elements and related parameters, or Pb isotopiccompositions have been changed between the Middle-Late Permian and the Early Triassic, andalso between the Middle Triassic and the Late Triassic, some particular tectonic implications could be inferred from these two considerable distinctions.(4)The nature and the tectonic settings of volcanic rocks since the Late Paleozoic have beenwell known by using the abundances and the ratios of high field strength elements, and the spiderdiagrams in the present thesis.The Permian continental tholeiites are formed in the continental extensional zone or initialrift in the within-plate, the Late Triassic tholeiite plots in the normal mid-ocean-ridge basalt field,the rhyodacites in Sangxiu Formation are formed in the stretched basin of continental margin.Apart from mentioned above, the Early Triassic basalts, the Middle Triassic basalts, and theMiddle Jurassic basalts, as well as the Late Jurassic basalts (Sangxiu Formation) and the EarlyCretaceous basalts are all characterized by transitional alkalic-tholeiitic characteristics, which plotin the rift field of continental margin.(5)The nature of magmatic source, the processes of magma and the evolutional characteristicsof volcanic rocks in the central belt of Tethyan Himalayas since the Late Paleozoic have beenrevealed preliminarily by systematically comparative analysis of geochemistry and Pb isotopiccomposition. The results show that three types of magmatic sources, which consist of lithosphericmantle, depleted asthenospheric mantle and enriched mantle plume, have been identified by thepresent study of geochemistry in detail. The sources of magma of volcanic rocks in different agesare the mixture of these three types of magma in different ratios, which mainly underwent partialmelting, as well as limited evolution by fractional crystallization.(6)Based on the analysis of petrology and geochemistry, petrogenesis of the volcanic rocks inSelong and Sangxiu Mountain has been studied in detail by the present thesis.Based on the study of geochemistry and field attitude of the Permian basalts, the presentthesis proposes that the magma that derived from the depleted asthenospheric mantle can tap theenriched lithospheric mantle due to the faults that cut through the stretched lithosphere, the finite,intermittent effusions of magma that has not experienced continental crustal contamination and ischaracterized by continental OIB-type could be occurred in the shore-shallow setting.According to the relationship and the geochemistry between rhyodacites and basalts,andesitic rocks in Sangxiu Formation, the present thesis puts forward a three-stage model ofpetrogenesis to interpret these volcanic activities: the first stage is the forming of alkaline basalticmagma, which is originated from the mixture of magma that experienced a lower degree of partialmelting in different ratios, the mixed magma derived from lithospheric mantle and from theOIB-type basaltic magma that induced by the activity of mantle plume. Subsequently, a decreasedgeothermal gradient and a subsidised interface between asthenosphere and lithosphere as aconsequence of this effusion of alkaline basaltic magma would led to the depth of magmaticsource become deeper than that of before. The second stage is the time that the forming of limitedandesitic magma, which originated from the early basaltic magma by fractional crystallization.The third stage is the time that the forming of rhyodacitic magma, which strongly differentiatedfrom the early basaltic and andesitic magma and experienced strikingly assimilation andcontamination by the upper crust in the process of ascend of magma.(7)Based on the detailed geochemical study of the volcanic rocks in the central belt ofTethyan Himalayas, the present thesis probes into the implications for these volcanic rocks sincethe Late Paleozoic. The main preliminarily conclusions as following: a. According to systematically comparative study of the Permian basalt in the western and theeastern belt of Tethyan Himalayas, as well as the newly paleomagnetic results by other researchers,the present thesis suggests that the initial opening of Yarlung Zangbo back-arc oceanic basin maybe happened early in the western and late in the eastern belt of Tethyan Himalayas.b. The depth of magmatic source of volcanic rocks is characterized by two cyclic changesfrom deep to low since the Late Paleozoic, this fact indicate that the lithosphere of northernmargin of Gondwana has been thinned strikingly by stretching from the Permian to the Cretaceous.The frequent, limited volcanic activities in the central belt of Tethyan Himalayas would attributeto the cooling of mantle materials beneath the stretched lithosphere by convecting.c. The Permian basaltic magmatic activity, which has neither trace element nor Pb isotopiccomposition relative to the MORB, is only one of the volcanic events that are response to thestretched lithosphere of northern margin of Gondwana at the end of the Late Paleozoic, the initialopening of Yarlung Zangbo back-arc oceanic basin could not inferred from this basaltic volcanicactivity.d. The present thesis proposes that the initial opening of Yarlung Zangbo back-arc oceanicbasin may be happened at the transition from the end of Permian to the Early Triassic, which couldbe inferred from the Pb isotopic compositions of the Early-Middle Triassic basalts are similar tothe MORB, and from the abruptly changes of geochemical parameters, and also from the stretcheddegree of lithosphere between the end of Permian and the Early Triassic. Yarlung Zangbo back-arcoceanic basin may be as a juvenile ocean at the Early-Middle Triassic deduced from the nature andthe spatial location of the Early-Middle Triassic basaltic activities.e. According to the distinct changes of geochemistry, Pb isotopic composition, and thegeochemical parameter, as well as the spatial location of volcanic activity, the paleomagneticresults of Himalayan block form the Middle Triassic to the Late Triassic, the present thesisindicates that the first rapid spreading of Yarlung Zangbo back-arc oceanic basin has beenoccurred at the transition of the Middle Triassic and the Late Triassic, and as a consequence of therapid spreading event, this oceanic basin opened completely and up to a certain scale, the studiedarea turns into a matured passive continental margin.f. The present thesis, which based on the regional geological setting, the geochemistry, therelationship in field section of the volcanic rocks, as well as the paleomagnetic results ofHimalayan block, points out that the middle-upper basalt in Sangxiu Formation may be as an"indicator" to indicates the second rapid spreading of Yarlung Zangbo back-arc oceanic basin,whereas the rhyodacites in the upper parts of Sangxiu Formation would indicate that the kineticdirection of Himalayan block has been transformed from south to north, and the Yarlung Zangboback-arc oceanic basin reaches to its greatest width and subsequently enters into slowly spreadingand closing stage. It is emphasizes that the volcanic rocks in Sangxiu Formation have recorded theprocess of the second rapid spreading of Yarlung Zangbo back-arc oceanic basin, and these rocksmay be as the indicators to reveal the information of tectonic evolution for this oceanic basin.(8)Based on the studies of the nature and the evolutional characteristics of magmatic sources,the dynamic mechanism of the volcanic activities in the central belt of Tethyan Himalayas sincethe Late Paleozoic has been discussed by the present thesis. It is suggests that the dynamicmechanism of these volcanic activities is induced by the passive rifting that may be enhanced dueto the upwelling of a deep mantle plume from a major thermal boundary layer within the deep mantle (660km discontinuity), the force source of passive rifting is from the mantle convection,which induced by the subduction southward of Bangong Co-Nujiang oceanic crust.(9)At the end of the present thesis, the author points out that the studied area may berepresents a relatively active volcanic rifted-type passive continental margin, which is differ fromthe traditionally stable passive continental margin. The studied area, which locates in thetransitional region between continental crust and oceanic crust, is in a rifted stage in the process ofevolution of the passive continental margin that is characterized by rifting, rift valley has not beenformed from the Late Triassic to the Early Cretaceous.