Query: magmatism quiangtang

title: Mesozoic mafic magmatism in North China: Implications for thinning and destruction of cratonic lithosphere
abstract: The North China Craton (NCC) has been thinned from >200 km to <100 km in its eastern part. The ancient subcontinental lithospheric mantle (SCLM) has been replaced by the juvenile SCLM in the Meoszoic. During this period, the NCC was destructed as indicated by extensive magmatism in the Early Cretaceous. While there is a consensus on the thinning and destruction of cratonic lithosphere in North China, it has been hotly debated about the mechanism of cartonic destruction. This study attempts to provide a resolution to current debates in the view of Mesozoic mafic magmatism in North China. We made a compilation of geochemical data available for Mesozoic mafic igneous rocks in the NCC. The results indicate that these mafic igneous rocks can be categorized into two series, manifesting a dramatic change in the nature of mantle sources at ~121 Ma. Mafic igneous rocks emplaced at this age start to show both oceanic island basalts (OIB) like trace element distribution patterns and depleted to weakly enriched Sr Nd isotope compositions. In contrast, mafic igneous rocks emplaced before and after this age exhibit both island arc basalts (IAB) like trace element distribution patterns and enriched Sr Nd isotope compositions. This difference indicates a geochemical mutation in the SCLM of North China at ~121 Ma. Although mafic magmatism also took place in the Late Triassic, it was related to exhumation of the deeply subducted South China continental crust because the subduction of Paleo Pacific slab was not operated at that time. Paleo Pacific slab started to subduct beneath the eastern margin of Eruasian continent since the Jurrasic. The subducting slab and its overlying SCLM wedge were coupled in the Jurassic, and slab dehydration resulted in hydration and weakening of the cratonic mantle. The mantle sources of ancient IAB like mafic igneous rocks are a kind of ultramafic metasomatites that were generated by reaction of the cratonic mantle wedge peridotite not only with aqueous solutions derived from dehydration of the subducting Paleo Pacific oceanic crust in the Jurassic but also with hydrous melts derived from partial melting of the subducting South China continental crust in the Triassic. On the other hand, the mantle sources of juvenile OIB like mafic igneous rocks are also a kind of ultramafic metasomatites that were generated by reaction of the asthenospheric mantle underneath the North China lithosphere with hydrous felsic melts derived from partial melting of the subducting Paleo Pacific oceanic crust. The subducting Paleo Pacific slab became rollback at ~144 Ma. Afterwards the SCLM base was heated by laterally filled asthenospheric mantle, leading to thinning of the hydrated and weakened cratonic mantle. There was extensive bimodal magmatism at 130 to 120 Ma, marking intensive destruction of the cratonic lithosphere. Not only the ultramafic metasomatites in the lower part of the cratonic mantle wedge underwent partial melting to produce mafic igneous rocks showing negative eNd(t) values, depletion in Nb and Ta but enrichment in Pb, but also the lower continent crust overlying the cratonic mantle wedge was heated for extensive felsic magmatism. At the same time, the rollback slab surface was heated by the laterally filled asthenospheric mantle, resulting in partial melting of the previously dehydrated rocks beyond rutile stability on the slab surface. This produce still hydrous felsic melts, which metasomatized the overlying asthenospheric mantle peridotite to generate the ultramafic metasomatites that show positive eNd(t) values, no depletion or even enrichment in Nb and Ta but depletion in Pb. Partial melting of such metasomatites started at ~121 Ma, giving rise to the mafic igneous rocks with juvenile OIB like geochemical signatures. In this context, the age of ~121 Ma may terminate replacement of the ancient SCLM by the juvenile SCLM in North China. Paleo Pacific slab was not subducted to the mantle transition zone in the Mesozoic as revealed by modern seismic tomography, and it was subducted at a low angle since the Jurassic, like the subduction of Nazca Plate beneath American continent. This flat subduction would not only chemically metasomatize the cratonic mantle but also physically erode the cratonic mantle. Therefore, the interaction between Paleo Pacific slab and the cratonic mantle is the first order geodynamic mechanism for the thinning and destruction of cratonic lithosphere in North China.

title: Evolution of Siderian juvenile crust to Rhyacian high Ba Sr magmatism in the Mineiro Belt, southern Sao Francisco Craton.
abstract: Plutonic rocks from the Mineiro Belt, Brazil record a delayed onset of the transition from TTG to sanukitoid type magmatism (high Ba Sr) starting during the Siderian magmatic lull when little juvenile magma was added to the continental crust. Rocks mostly belong to the calc alkaline series, meta to peraluminous and originally "I type" meaning that oxidized magmas were formed by partial melting of subducted material. The temporal distribution and apparent secular changes of the magmas are consistent with the onset of subduction driven plate tectonics due to an increase of the subduction angle and opening of the mantle wedge. New isotopic analyses (Sm Nd whole rock and Lu Hf in zircon) corroborate the restricted juvenile nature of the Mineiro Belt and confirm the genetic link between the Lagoa Dourada Suite, a rare ca. 2350 Ma high Al tonalite trondhjemite magmatic event, and the sanukitoid type ca.  Several plutons in the region have ages close to 2130 Ma but are distinguished by the lower concentration of compatible elements in the juvenile high Ba Sr suite.

title: Subduction initiation without magmatism: The case of the missing Alpine magmatic arc
abstract: Models of orogens identify subduction of oceanic crust as the key mechanism leading to continental collision. Such models, based inter alia on thermobarometric and geochronological evidence preserved in high pressure metamorphic rocks and subduction related magmatism, have been used to explain the convergence of Europe and Adria in the Cretaceous Cenozoic and the subsequent Alpine orogen. Here, we review the metamorphic, igneous, and sedimentary record of the past 300 Ma of the Alpine orogen to show that there is no evidence of igneous activity during subduction initiation and prograde high pressure metamorphism, leading to an ~50 Ma hiatus in magmatism, or "arc gap." The closure of rift basins forming the Piemont Liguria ocean did not follow a classical Wadati Benioff type subduction. Instead, subduction initiation at passive margins allowed for the accretion of the hydrated portion of the subducting plate within an orogenic wedge as subduction of dry subcontinental lithosphere inhibited magmatism during subduction initiation and ocean closure.  Recycling of oceanic lithosphere through subduction is a fundamental process governing plate tectonics, continental collision, and arc magmatism (Cloos, 1993) The mechanisms allowing for the initiation of new subduction zones are related to either spontaneous or induced subduction and are thought to occur predominantly within weaknesses in oceanic lithosphere such as transform faults and oceanic detachments (e.g. Stern and Gerya, 2017) Well documented examples of subduction initiation in mature oceanic settings (Neotethys supra subduction zone ophiolites and Izu Bonin Mariana arc) have revealed that subduction initiation is characterized by an initial stage of upper plate extension and tholeiitic to boninitic magmatism (e.g. Shervais, 2001; Maffione et al. 2017) Once initiated, partial eclogitization and densification of the subducting oceanic lithosphere results in a slab pull mechanism representing a driving force for self sustaining subduction and ocean closure (e.g. Cloos, 1993) In addition, dehydration of the oceanic crust drives flux melting of the overlying mantle wedge, resulting in predominantly "calc alkaline" (cf. Arculus, 2003) magmatism (e.g. Grove et al. 2012) Therefore, calc alkaline magmatism and low temperature high pressure metamorphic rocks are interpreted as strong evidence of paleo subduction zones (e.g. Stern, 2005) Evidence of (ultra)high pressure continental and oceanic fragments in the European Alpine orogen (e.g. Chopin, 1984; Reinecke, 1998)  has led to models implying the subduction of a 500 1000 km wide Jurassic Piemont Liguria ocean during Cretaceous Cenozoic convergence (e.g. Stampfli et al. 1998; Handy et al. 2010) The Alpine orogen, however, upon subduction initiation at ca. 85 100 Ma (Rosenbaum and Lister, 2005; Handy et al. 2010; Zanchetta et al. 2012) lacks the distinctive characteristics of subduction initiation recorded in Neotethyan ophiolites  Crucially, unlike Neotethyan ophiolites, preserved alpine ophiolites indicate that the Piemont Liguria ocean was not floored by mature ocean crust but formed predominantly of basins floored by exhumed subcontinental mantle, similar to magma poor Iberia Newfoundland oceancontinent transition zones (OCTs) (e.g. Manatschal and Muntener, 2009; Mohn et al. 2011; Picazo et al. 2016) The complex architectures of these basins are thought to play a major role in the mechanisms controlling the closure of the Piemont Liguria ocean (e.g. Tugend et al. 2014) Here we compile igneous, sedimentary, and metamorphic geochronological data of the past 300 Ma encompassing the Alpine orogeny  and show that there is no magmatic record of subduction initiation and closure of the Piemont Liguria ocean in the Alps. We discuss potential reasons for subduction without magmatism and propose alternative mechanisms of subduction initiation occurring at passive margins.  McCarthy, A. et al. 2018, Subduction initiation without magmatism: The case of the missing Alpine magmatic arc: Geology, v. 46, p. 1059 1062, https:/doi.org/10.1130/G45366.1 Manuscript received 10 July 2018 Revised manuscript received 8 October 2018 Manuscript accepted 11 October 2018 https:/doi.org/10.1130/G45366.1 (c) 2018

title: Dynamics of episodic Late Cretaceous Cenozoic magmatism across Central to Eastern Anatolia: New insights from an extensive geochronology compilation
abstract: We compiled geochronology data from 87 published studies within the Anatolia orogen (32.5degE 44degE) to investigate the spatial and temporal patterns of continental magmatism during the final stages of Neotethys Ocean closure. The number and diversity of studies compiled here collectively provide a thorough characterization of magmatism >700 ages) in the Anatolia orogen since the Late Cretaceous (ca. 100 Ma) Our new compilation reveals that magmatism was episodic and occurred in three distinct magmatic episodes punctuated by two orogen wide magmatic lulls. We used regional scale insights into the timing, location, composition, and evolution of magmatism revealed by our compilation to evaluate the tectonic and geodynamic processes responsible for each widespread magmatic lull, and to test and refine existing geodynamic models for Anatolia. We interpret the first orogen wide magmatic lull (ca. 72 58 Ma) to have been the result of Maastrichtian to Paleocene collision of the Kirsehir and Anatolide Tauride blocks with the Pontides arc along the Izmir Ankara Erzincan suture zone and synchronous collision of the Bitlis Puturge massif with the southern margin of the Anatolide Tauride blocks along the Bitlis suture zone. Magmatic quiescence during the second magmatic lull (ca. 40 20 Ma) was variably related to terminal subduction and Arabia slab break off along the Bitlis suture zone in the east, and Cyprus slab flattening due to postcollisional southward retreat of the Cyprus trench in the west, each triggered by middle to late Eocene Arabia collision. Postcollisional Neogene Quaternary magmatism was most likely caused by lithospheric delamination and slab tearing/rollback in the Eastern and Central Anatolia volcanic provinces, respectively.

title: Petrological, geochemical, isotopic, and geochronological constraints for the Late Devonian Early Carboniferous magmatism in SW Gondwana (27 32degLS) an example of geodynamic switching
abstract: We report a study integrating 13 new U Pb LA MC ICP MS zircon ages and Hf isotope data from dated magmatic zircons together with complete petrological and whole rock geochemistry data for the dated granitic rocks. Sample selection was strongly based on knowledge reported in previous investigations. Latest Devonian Early Carboniferous granite samples were collected along a transect of 900 km, from the inner continental region (present day Eastern Sierras Pampeanas) to the magmatic arc (now Western Sierras Pampeanas and Frontal Cordillera) Based on these data together with ca. 100 published whole rock geochemical analyses we conclude that Late Devonian Early Carboniferous magmatism at this latitude represents continuous activity (ranging from 322 to 379 Ma) on the pre Andean margin of SW Gondwana, although important whole rock and isotopic compositional variations occurred through time and space. Combined whole rock chemistry and isotope data reveal that peraluminous A type magmatism started in the intracontinental region during the Late Devonian, with subsequent development of synchronous Carboniferous peraluminous and metaluminous A type magmatism in the retro arc region and calc alkaline magmatism in the western paleomargin. We envisage that magmatic evolution was mainly controlled by episodic fluctuations in the angle of subduction of the oceanic plate (between flat slab and normal subduction) supporting a geodynamic switching model. Subduction fluctuations were relatively fast (ca. 7 Ma) during the Late Devonian and Early Carboniferous, and the complete magmatic switch off and switch on process lasted for 57 Ma. Hf TDM values of zircon (igneous and inherited) from some Carboniferous peraluminous A type granites in the retro arc suggest that Gondwana continental lithosphere formed during previous orogenies was partly the source of the Devonian Carboniferous granitic magmas, thus precluding the generation of the parental magmas from exotic terranes.

title: A review of the Famatinian Ordovician magmatism in southern South America: evidence of lithosphere reworking and continental subduction in the early proto Andean margin of Gondwana
abstract: Along the proto Pacific margin of Gondwana, from Venezuela to northeastern Patagonia, the Early Middle Ordovician Famatinian orogeny was the first orogenic event following assembly of the supercontinent. Previous isotope studies of the igneous and (meta )sedimentary rocks of southwestern Gondwana yield ambiguous implications for the role of juvenile mantle addition during the early crustal growth at the supercontinental margin. To interpret the geological and tectonic evolution of the orogen and the magma sources in different episodes we look at evidence from a large area of southern South America, including the 700 x 600 km type sector of the orogen in the Sierras Pampeanas (27deg 33degS) the Precordillera, and northeastern Patagonia. Previous geological, geochemical and geochronological results are reviewed together with new U Pb SHRIMP crystallization ages, 177Hf/176Hf and 18O/16O data for dated zircon, and whole rock Sr and Nd isotope compositions. Four geological domains are recognized in the Sierras Pampeanas (Western, Central, Eastern and Foreland Famatinian domains) Magmatism is mostly restricted to the interval 463 4 to 486 7 Ma, with the most intense period of emplacement between 468 and 472 Ma constituting a magmatic flare up. Granitoid emplacement in both northeastern Patagonia and the Cordon de Lila (Puna Altiplano, Chile) was effectively synchronous with that in the Sierras Pampeanas, defining a continuous belt. Combined geochemical and isotopic data (whole rock Sr, Nd; Hf, O in zircon) indicate that the source of calcic metaluminous suites is the subcontinental lithosphere both mantle and mafic lower crust with variable contamination by the Early Paleozoic metasedimentary country rocks. The lithospheric mantle involved is assumed to underlie the outcropping 1330 1030 Ma age basement of the Western Domain, which exhibits tectonic characteristics of active continental margin in the north and oceanic arc back arc in the south. The latter sector is the potential source of some minor Famatinian igneous rocks with less evolved isotopic compositions, although a restricted asthenospheric addition cannot be discarded in this case. Minor peraluminous granites are spatially associated with the metaluminous sequence, but major highly peraluminous batholiths occur on the eastern flank of the Central Domain. Field relations and geochemical/isotopic evidence indicate that the most obvious source of these crustal melts was the very thick post early Cambrian metasedimentary sequence comprising the host country rocks. Episodic tectono magmatic evolution of the Famatinian magmatic belt in two overlapping stages is invoked to explain different characteristics in the four recognized domains in the type sector: ca. 474 486? Ma, roll back stage. This is a mainly extensional interval involving asthenospheric upwelling and thinning of the subcontinental mantle; full development of the marine ensialic basins and early emplacement of both metaluminous granites and highly peraluminous batholiths in the Central and Eastern Famatinian domains. Trondhjemite plutons with an adakitic signature were emplaced in the Foreland Domain ca. 468 472 Ma, slab break off stage. Steepening of the oceanic slab and arc migration to the southwest ended with slab break off due to subduction of continental crust during continental collision with the Precordillera terrane. This stage produced voluminous metaluminous magmatism at the western edge of the Central Domain (the flare up episode) K bentonites in the Precordillera, leucogranites in the Western Domain and scattered metaluminous and peraluminous plutons in all Famatinian domains. Both slab roll back and break off stages developed during a high T regime typical of hot orogens. Although asthenospheric mantle was a necessary heat source for lithospheric melting, its material contribution to the growth of Early Paleozoic crust was apparently very minor. Recycling of Mesoproterozoic lithosphere, including the subcontinental mantle, coupled with crustal melting of Early Paleozoic metasedimentary sequences, accounts for most Famatinian magmatism. Comparable results from the Central Andes and East Antarctica confirm that the early stages of the Terra Australis orogen in SW Gondwana were dominated by lithospheric reworking processes.

title: Continental crust formation on early Earth controlled by intrusive magmatism
abstract: The global geodynamic regime of early Earth, which operated before the onset of plate tectonics, remains contentious. As geological and geochemical data suggest hotter Archean mantle temperature and more intense juvenile magmatism than in the present day Earth, two crust mantle interaction modes differing in melt eruption efficiency have been proposed: the Io like heat pipe tectonics regime dominated by volcanism and the "Plutonic squishy lid" tectonics regime governed by intrusive magmatism, which is thought to apply to the dynamics of Venus. Both tectonics regimes are capable of producing primordial tonalite trondhjemite granodiorite (TTG) continental crust but lithospheric geotherms and crust production rates as well as proportions of various TTG compositions differ greatly, which implies that the heat pipe and Plutonic squishy lid hypotheses can be tested using natural data. Here we investigate the creation of primordial TTG like continental crust using self consistent numerical models of global thermochemical convection associated with magmatic processes. We show that the volcanism dominated heat pipe tectonics model results in cold crustal geotherms and is not able to produce Earth like primordial continental crust. In contrast, the Plutonic squishy lid tectonics regime dominated by intrusive magmatism results in hotter crustal geotherms and is capable of reproducing the observed proportions of various TTG rocks. Using a systematic parameter study, we show that the typical modern eruption efficiency of less than 40 per cent leads to the production of the expected amounts of the three main primordial crustal compositions previously reported from field data (low medium and high pressure TTG) Our study thus suggests that the pre plate tectonics Archean Earth operated globally in the Plutonic squishy lid regime rather than in an Io like heat pipe regime.

title: Neoproterozoic amalgamation between Yangtze and Cathaysia blocks: The magmatism in various tectonic settings and continent arc continent collision
abstract: According to the temporal spatial distribution of Neoproterozoic igneous rocks and relative rocks in South China, including ophiolites, arc volcanic and intrusive rocks and subsequent bimodal magmatism, we identified the presence of a Neoproterozoic intra oceanic arc, continent arc continent collision and three tectono magmatic zones between the Yangtze Block and Cathaysia Block. We have also unraveled the amalgamation and tectono magmatic histories between the Yangtze and Cathaysia blocks: At ~1000 860 Ma, northwestward ocean ocean subduction and southeastward ocean continent subduction resulted in the intra oceanic arc magmatism and active continental margin magmatism in the Cathaysia Block respectively. At ~860 825 Ma, the steepening subduction caused development of back arc basin in the intra oceanic arc zone and the slab rollback induced the arc and back arc magmatism in the Cathaysia Block. Meanwhile, a shallow dip northwestward ocean continent subduction formed active continental margin magmatism in the Yangtze Block. At ~825 805 Ma, the continent arc continent collision and final amalgamation between the Yangtze and Cathaysia blocks yielded the Jiangnan Orogen. At ~805 750 Ma, the Jiangnan Orogen collapsed, and the Nanhua rift basin formed. Our study also rules out any Grenvillian Orogenic event and mantle plume activity in South China and indicates a marginal position of South China in the Rodinia supercontinent.

title: The Cenozoic magmatism of East Africa: Part I Flood basalts and pulsed magmatism
abstract: Cenozoic magmatism in East Africa results from the interplay between lithospheric extension and material upwelling from the African Large Low Shear Velocity Province (LLSVP) The modern focusing of East African magmatism into oceanic spreading centers and continental rifts highlights the modern control of lithospheric thinning in magma generation processes, however the widespread, and volumetrically significant flood basalt events of the Eocene to Early Miocene suggest a significant role for material upwelling from the African LLSVP. The slow relative motion of the African plate during the Cenozoic has resulted in significant spatial overlap in lavas derived from different magmatic events. This complexity is being resolved with enhanced geochronological precision and a focus on the geochemical characteristics of the volcanic products. It is now apparent that there are three distinct pulses of basaltic volcanism, followed by either bimodal lavas or silicic volcanic products during this period: (A) Eocene Initial Phase from 45 to 34 Ma. This is a period of dominantly basaltic volcanism focused in Southern Ethiopia and Northern Kenya (Turkana) (B) Oligocene Traps phase from 33.9 to 27 Ma. This period coincides with a significant increase in the aerial extent of volcanism with broadly age equivalent 1 to 2 km thick sequences of dominantly basalt centered on the NW Ethiopian Plateau and Yemen, (C) Early Miocene resurgence phase from 26.9 to 22 Ma. This resurgence in basaltic volcanism is seen throughout the region at ca. 24 23 Ma, but is less volumetrically significant than the prior two basaltic pulses. With our developing understanding of the persistence of LLSVP anomalies within the mantle, I propose that the three basaltic pulses are ostensibly manifestations of the same plume lithosphere interaction, requiring revision to the duration, magmatic extent, and magma volume of the African Arabian Large Igneous Province.

title: Collision vs. subduction related magmatism: two contrasting ways of granite formation and implications for crustal growth
abstract: Earth's continental crust is dominantly made of buoyant, felsic igneous material (granitoids) that were ultimately extracted from the mantle as a result of Earth's differentiation. Since felsic melts are not in chemical equilibrium with the mantle, they can originate either from melting of older crustal lithologies, or from differentiation of a primitive mantle melt; only the latter case will contribute to crustal growth. To understand the mechanisms of continental crust growth and differentiation through time, it is therefore necessary to unravel the respective contribution of these two different mechanisms in the genesis of granitoid suites. In modern Earth, granitoids are chiefly generated in convergent plate boundaries (subduction and collision) This paper examines the granitic suites in a late collision environment, the Variscan French Massif Central (FMC) and compares them with the suites found in an oceanic arc. We therefore describe, and compare, two end members sites of granite generation. In the FMC, several main types of granites are described. Muscovite and Cordierite bearing Peraluminous Granites (resp. MPG and CPG) contain large amounts of inherited zircons, and their chemistry demonstrates that their sources were older crustal material (resp. Metasediments and metaigneous) On the other hand, Potassic Calc alkaline Granites (KCG) associated to potassic diorites (vaugnerites) do not contain inherited zircons, and ultimately derive from the vaugnerites. The vaugnerites in turns form by partial melting of a mantle contaminated by the regional crust. Therefore, although they are isotopically similar to the crust, the KCG are net contributors to crustal growth. Thus we conclude that although late orogenic settings are dominated by crustal melting and recycling, they may be sites of net crustal growth, even though this is not visible from isotopes only. In contrast, arc granitoids are purely or almost purely mantle derived. However, the preservation potential of arcs is much smaller than the preservation of late orogenic domains, such that at the scale of a whole orogenic belt, late orogenic magmatism is probably as important as arc magmatism. Finally, we speculate that the situation may have been similar in the Archaean, or even more skewed towards late orogenic sites (or similar environments, dominated by melting of a altered mafic protocrust) owing to the hotter mantle and less stable subductions during that period.