Magmatic Evolution Of The Ando Microcontinent In The Tibet Plateau

The Tibet Plateau was built on a tectonic collage created by sequential accretion of several microcontinents,accretionary belts,and island arcs onto the southern margin of Eurasia since the early Paleozoic.The Ando Block is the oldest terrane with Precambrain basement in the Plateau interior.It was bounded between Lhasa and Qiangtang terranes as an east-west-trending lense.In the Ando Block,there are many Neoproterozoic to Early Mesozoic magmatic rocks that recorded its geological evolution in different periods.In this research,we conducted detailed field investigations and petrography observations in the Ando Block,and also presented new zircon U-Pb ages and Lu-Hf isopotic data,whole-rock geochemical and Sr-Nd isotopic data for the Early Neoproterozoic,Early Paleozoic and Early Mesozoic magatic rocks in order to constrain the rock types,intrusive ages,petrogenesises and tectonic settings.This will help us to reveal the geological evolution and discuss the formation,reworking and growth of the Ando continental crust.Our zircon U-Pb dating for the Neoproterozoic granitic gneisses yielded an emplaced age at 8⁸⁷ Ma.The granitic gneisses have high SiO2?65.56⁷9.82 wt.%?,A/CNK?1.39¹.68?,CIPW?2.74⁵.10 wt.%?.Combined with the occurrance of aluminium-rich mineral?e.g.,muscovite?,these geochemical features indicate that the protolith of the orthogneiss is strong peraluminous S-type granite.The orthogneiss display low CaO/?MgO+FeO^T?,Al2O3/TiO2,Rb/Sr,Rb/Ba,and high CaO/Na2O?>0.3?and show enriched Nd and Hf isotopic compositions??Nd?t?=-5.51 ^-2.76;?Hf?t?=-15.79 4.56?,suggesting the protolith of orthogneiss were most likely produced by partial melting of the metapelite.This interpretation is in agreement with the commonly occurred inherited zircons.The high MgO,Cr and Ni contents likely indicate mantle-derived materials or juvenile crust components in the magma source.Zircon U-Pb dating of the Early Paleozoic granitic gneisses in the Ando Block demonstrated that the rocks were cystaliazed at 484-500 Ma.They show a large variation of SiO2(68.46^<sup>87.52 %)and have low Na2O/K2O?<1?,P2O5?<0.1 %?and CIPW?0.62².07 wt.%?.These geochemical features,combined with the existence of hornblende,indicate that the protolith of orthogneiss is I-type granite.The rocks display enriched Nd and Hf isotopic compositions??Nd?t?=-10.98^-4.54;?Hf?t?=-10.38¹.31?,indicating that there were derived from the partial melting of Ando basement.However,they have higher ?Nd?t?and ?Hf?t?values than those of the Neoproterozoic orthogneiss.Because of the contemporaneous mafic enclaves,the protolith of the Early Paleozoic orthogneisses were unlikely produced only by partial melting of Ando basement.We therefore suggest that mantle-derived materials were involved in the formation of these magmatic rocks.Some samples have high SiO2,FeO^T/MgO and?K2O+Na2O?/CaO,probably indicating that some of these samples have experienced fractional crystallization during magma evolution.Based on our studies,the Mesozoic intrusive rocks mainly consist of four types,including MORB-type gabbros,high-Mg diorites,calc-alkaline diorites and granodiorites.Our zircon U-Pb dating yielded similar crystallization ages at 174-177 Ma for the four types in the Ando Block,but the geochemical data reveal different petrogenesis:?1?Gabbro: The Ando gabbros show relatively flat chondrite-normalized REE patterns??La/Yb?N= 0.70¹.13?and depleted ?Nd?t??4.40⁵.50?and ?Hf?t??10.33¹5.55?,isotopic composition similar to the Bangong-Nujiang ophiolite,indicating an origin from asthenosphere mantle.The gabbro are enriched in LILE?e.g.,K,Rb,Pb?,depleted in HFSE?e.g.,Nb,Ta,Ti?,and have high Ba/La?8¹5?,Ba/Th?29¹50?,U/Th?0.57².73?,low Th/Yb?0.05⁰.33?rates.This suggests aqueous fluids play an important role in their formations.The geochemical features,combined with the occurrence of a large amount of aqueous minerals,such as hornblend,imply that the enriched lithospheric mantle metasomatized by aqueous fluids has participated in the formation of the studied gabbros.Therefore,the gabbros were most likely derived from the upwelling asthenosphere derived melts metasomatized by enriched lithospheric mantle.the gabbros have low and constant Sm/Yb and Dy/Yb,which suggest that the gabbros were derived from partial melting of a spinel-facies lherzolite mantle,indicating a relatively shallow depth of melting?<80 km?.?2?High-Mg diorite: The diorite samples have high MgO?8.30¹0.24 wt.%?and Mg#?70⁷4?,Cr?400⁵47 ppm?and Ni?120¹52 ppm?,low HREE,Sr/Y and La/Yb ratios,which are quite analogous to those of sanukites.Combining with high K2O?K2O/Na2O=3.19⁵.04?,Ba?>1000 ppm?,La/Sm,Th/Yb,Ba/La,and low Ba/Th,Sr/Nd rates,enriched ?Nd?t??-10.80 ^-10.50?and ?Hf?t??-11.95 ^-6.47?suggest that the high-Mg diorites were probably generated by the interaction of subducted oceanic sediment-derived melts and mantle peridotites.?3?Calc-alkaline diorite: Comparing with the high-Mg diorites,the diorites have relatively low MgO,Mg#,Cr,Ni,and high FeO^T values.Their ?Nd?t?and initial ⁸⁷Sr/86 Sr isotopic ratio are slightly higher than the Ando basement??Nd?t?=-7.5^-7.3;?Hf?t?=-0.04^-12.33?.These features,therefore,suggest that the diorites were most likely produced by partial melting of the Ando basement rocks with significant input of mantle components.?4?Calc-alkaline granodiorite: Their enriched Nd??Nd?t?=-10.80^-10.60?and initial ⁸⁷Sr/86Sr((86Sr/⁸⁷Sr)i= 0.7303⁰.7306)isotopic ratios are close to the Ando basement(?Nd?t?=-7.49^-9.56,(86Sr/⁸⁷Sr)i= 0.7268⁰.7383),indicating that the granodiorites were possibly derived from partial melting of the Ando basement rocks.Combined with the previous published date,we suggest the three phases of Ando magmatic rocks were generated at continental marginal arc,namely subduction-accretion orogeny system.The Ando continental crust was produced in the early Neoproterozoic,then experienced coevally rework and growth both in the early Paleozoic and the early Mesozoic magmatic rocks shows the Ando continental crust,once again,experienced rework and growth.