Enrichment Nature of Ultrapotassic Rocks in Southern Tibet Inherited from their Mantle Source

Post-collisional ultrapotassic rocks (UPRs) in the Tibetan Plateau exhibit extreme enrichment in incompatible elements and radiogenic isotopes. Such enrichment is considered to be either inherited from a mantle source or developed during crustal evolution. In this study, to solve this debate we combined mineral textures and in situ geochemical composition of clinopyroxene phenocrysts in UPRs from southern Tibet to reveal their crustal evolution, enrichment cause and constrain metasomatism in their mantle source. Results show that the UPRs experienced an array of crustal processes, i.e., fractional crystallization, mixing, and assimilation. Fractional crystallization is indicated by decreases in Mg# and Ni and enrichment in incompatible elements (e.g. rare earth element (REE), Sr, Zr) toward the rims of normally zoned clinopyroxene phenocrysts (type-I). Magma mixing is evidenced by the presence of some clinopyroxene phenocrysts (type-II, -III) showing disequilibrium textures (e.g. reversed and overgrowth zoning), but in situ Sr isotope and trace element analysis of those disequilibrium zones indicate that late-stage recharged mafic magmas are depleted (87Sr/86Sr: 0.70659–0.71977) compared with the primitive ultrapotassic magmas (87Sr/86Sr: 0.70929–0.72553). Assimilation is revealed by the common presence of crustal xenoliths in southern Tibetan UPRs. Considering the much lower 87Sr/86Sr values (0.707759–0.709718) and incompatible element contents of these crustal xenoliths relative to their host UPRs, assimilation should have resulted in geochemical depletion of southern Tibetan UPRs rather than enrichment. The diluting impact of both assimilation and mixing is also supported by the modeling results based on the EC-E′RAχFC model combining the growth history of clinopyroxene. Trace elements ratios in clinopyroxenes also imply that the mantle source of southern Tibetan UPRs suffered an enriched and carbonatite-dominated metasomatism. Thus, we conclude that enrichment of southern Tibetan UPRs was inherited from the mantle source.

[1]  Weikai Li,et al.  Supplemental Material: Redox state of southern Tibetan upper mantle and ultrapotassic magmas , 2020, Geology.

[2]  M. Cooper,et al.  A short, sharp pulse of potassium-rich volcanism during continental collision and subduction , 2019, Geology.

[3]  Yongjun Lu,et al.  Redox-controlled generation of the giant porphyry Cu–Au deposit at Pulang, southwest China , 2019, Contributions to Mineralogy and Petrology.

[4]  Yongjun Lu,et al.  Miocene Ultrapotassic, High‐Mg Dioritic, and Adakite‐like Rocks from Zhunuo in Southern Tibet: Implications for Mantle Metasomatism and Porphyry Copper Mineralization in Collisional Orogens , 2018 .

[5]  Zhihui Cheng,et al.  Post-collisional ultrapotassic rocks and mantle xenoliths in the Sailipu volcanic field of Lhasa terrane, south Tibet: Petrological and geochemical constraints on mantle source and geodynamic setting , 2017 .

[6]  S. Foley,et al.  Potassium-rich magmatism from a phlogopite-free source , 2017 .

[7]  Shan Gao,et al.  Accurate Determination of Sr Isotopic Compositions in Clinopyroxene and Silicate Glasses by LA‐MC‐ICP‐MS , 2016 .

[8]  D. DePaolo,et al.  Identifying mantle carbonatite metasomatism through Os–Sr–Mg isotopes in Tibetan ultrapotassic rocks. , 2015 .

[9]  M. Wilson,et al.  Post-collisional Ultrapotassic Mafic Magmatism in South Tibet: Products of Partial Melting of Pyroxenite in the Mantle Wedge Induced by Roll-back and Delamination of the Subducted Indian Continental Lithosphere Slab , 2015 .

[10]  D. Prelević,et al.  Magmatic Response to Slab Tearing: Constraints from the Afyon Alkaline Volcanic Complex, Western Turkey , 2015 .

[11]  Yongjun Lu,et al.  High-Mg diorite from Qulong in southern Tibet: Implications for the genesis of adakite-like intrusions and associated porphyry Cu deposits in collisional orogens , 2015 .

[12]  T. Harrison,et al.  Postcollisional potassic and ultrapotassic rocks in southern Tibet: Mantle and crustal origins in response to India-Asia collision and convergence , 2014 .

[13]  Yue-heng Yang,et al.  Re-evaluation of interferences of doubly charged ions of heavy rare earth elements on Sr isotopic analysis using multi-collector inductively coupled plasma mass spectrometry , 2014 .

[14]  M. Wilson,et al.  Post-collisional, K-rich mafic magmatism in south Tibet: constraints on Indian slab-to-wedge transport processes and plateau uplift , 2013, Contributions to Mineralogy and Petrology.

[15]  D. Jacob,et al.  Recycling plus: A new recipe for the formation of Alpine–Himalayan orogenic mantle lithosphere , 2013 .

[16]  R. Romer,et al.  Ultrapotassic Mafic Rocks as Geochemical Proxies for Post-collisional Dynamics of Orogenic Lithospheric Mantle: the Case of Southwestern Anatolia, Turkey , 2012 .

[17]  M. Palmer,et al.  Petrogenesis of the Neogene volcanic units in the NE–SW-trending basins in western Anatolia, Turkey , 2012, Contributions to Mineralogy and Petrology.

[18]  Wenjin Zhao,et al.  Tibetan plate overriding the Asian plate in central and northern Tibet , 2011 .

[19]  Fu-Yuan Wu,et al.  Fragments of hot and metasomatized mantle lithosphere in Middle Miocene ultrapotassic lavas, southern Tibet , 2011 .

[20]  Z. Hou,et al.  The Lhasa Terrane: Record of a microcontinent and its histories of drift and growth , 2010 .

[21]  C. Macpherson,et al.  Geochemical and Sr–O isotopic constraints on magmatic differentiation at Gede Volcanic Complex, West Java, Indonesia , 2010 .

[22]  A. Peccerillo,et al.  Interaction between ultrapotassic magmas and carbonate rocks: Evidence from geochemical and isotopic (Sr, Nd, O) compositions of granular lithic clasts from the Alban Hills Volcano, Central Italy , 2010 .

[23]  F. Holtz,et al.  Experimental constraints on ultrapotassic magmatism from the Bohemian Massif (durbachite series, Czech Republic) , 2010 .

[24]  P. Robinson,et al.  Geochemical and Sr–Nd–Pb–O isotopic compositions of the post-collisional ultrapotassic magmatism in SW Tibet: Petrogenesis and implications for India intra-continental subduction beneath southern Tibet , 2009 .

[25]  M. Mattei,et al.  Potassic and ultrapotassic magmatism in the circum-Tyrrhenian region: Significance of carbonated pelitic vs. pelitic sediment recycling at destructive plate margins , 2009 .

[26]  S. Cronin,et al.  Rapid timescales of differentiation and evidence for crustal contamination at intra-oceanic arcs: Geochemical and U–Th–Ra–Sr–Nd isotopic constraints from Lopevi Volcano, Vanuatu, SW Pacific , 2008 .

[27]  J. Woodhead,et al.  Strontium Isotope Analysis of Kimberlitic Groundmass Perovskite via LA‐MC‐ICP‐MS , 2007 .

[28]  D. Morgan,et al.  Microsampling and Isotopic Analysis of Igneous Rocks: Implications for the Study of Magmatic Systems , 2007 .

[29]  K. Herwig,et al.  MPI‐DING reference glasses for in situ microanalysis: New reference values for element concentrations and isotope ratios , 2006 .

[30]  E. Draganits,et al.  Himalayan architecture constrained by isotopic tracers from clastic sediments , 2005 .

[31]  Xiaoming Qu,et al.  Origin of adakitic intrusives generated during mid-Miocene east–west extension in southern Tibet , 2004 .

[32]  S. Kelley,et al.  Nature of the source regions for post-collisional, potassic magmatism in Southern and Northern Tibet from geochemical variations and inverse trace element modelling , 2004 .

[33]  F. Spera,et al.  Open-System Magma Chamber Evolution: an Energy-constrained Geochemical Model Incorporating the Effects of Concurrent Eruption, Recharge, Variable Assimilation and Fractional Crystallization (EC-E′RAχFC) , 2003 .

[34]  L. Ding,et al.  Cenozoic Volcanism in Tibet: Evidence for a Transition from Oceanic to Continental Subduction , 2003 .

[35]  F. Ryerson,et al.  New clinopyroxene-liquid thermobarometers for mafic, evolved, and volatile-bearing lava compositions, with applications to lavas from Tibet and the Snake River Plain, Idaho , 2003 .

[36]  A. Hofmann,et al.  Neodymium and Strontium Isotope Data for USGS Reference Materials BCR‐1, BCR‐2, BHVO‐1, BHVO‐2, AGV‐1, AGV‐2, GSP‐1, GSP‐2 and Eight MPI‐DING Reference Glasses , 2003 .

[37]  A. Woodland,et al.  The distribution of lithium in peridotitic and pyroxenitic mantle lithologies — an indicator of magmatic and metasomatic processes , 2000 .

[38]  R. Schuster,et al.  Post-Collisional Potassic and Ultrapotassic Magmatism in SW Tibet: Geochemical and Sr-Nd-Pb-O Isotopic Constraints for Mantle Source Characteristics and Petrogenesis , 1999 .

[39]  D. Green,et al.  Carbonatite metasomatism in the southeastern Australian lithosphere , 1998 .

[40]  N. Rogers,et al.  Post-collision, Shoshonitic Volcanism on the Tibetan Plateau: Implications for Convective Thinning of the Lithosphere and the Source of Ocean Island Basalts , 1996 .

[41]  M. A. Morrison,et al.  Ultrapotassic Magmas along the Flanks of the Oligo-Miocene Rio Grande Rift, USA: Monitors of the Zone of Lithospheric Mantle Extension and Thinning Beneath a Continental Rift , 1993 .

[42]  C. Pin,et al.  Evaluation of a strontium-specific extraction chromatographic method for isotopic analysis in geological materials , 1992 .

[43]  A. Peccerillo,et al.  Potassic and ultrapotassic magmas and their origin , 1992 .

[44]  R. Goldfarb,et al.  Generation of postcollisional porphyry copper deposits in southern Tibet triggered by subduction of the Indian continental plate , 2016 .

[45]  T. Harrison,et al.  Zircon xenocrysts in Tibetan ultrapotassic magmas: Imaging the deep crust through time , 2014 .

[46]  M. Tiepolo,et al.  Crystal recycling in the steady-state system of the active Stromboli volcano: a 2.5-ka story inferred from in situ Sr-isotope and trace element data , 2011, Contributions to Mineralogy and Petrology.

[47]  G. Giordano,et al.  Shoshonite and sub-alkaline magmas from an ultrapotassic volcano: Sr–Nd–Pb isotope data on the Roccamonfina volcanic rocks, Roman Magmatic Province, Southern Italy , 2009 .

[48]  Keith Putirka,et al.  Thermometers and Barometers for Volcanic Systems , 2008 .

[49]  Q. Zhang,et al.  Tibetan tectonic evolution inferred from spatial and temporal variations in post-collisional magmatism , 2005 .

[50]  F. Siena,et al.  Carbonatite Metasomatism of the Oceanic Upper Mantle: Evidence from Clinopyroxenes and Glasses in Ultramafic Xenoliths of Grande Comore, Indian Ocean , 1999 .

[51]  D. Günther,et al.  Inter-laboratory note. Laser ablation inductively coupled plasma mass spectrometric transient signal data acquisition and analyte concentration calculation , 1996 .

[52]  W. McDonough,et al.  Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes , 1989, Geological Society, London, Special Publications.