| The East Kunlun Orogenic Belt (EKOB), a giant granitoid magmatic belt in Northern Qinghai-Tibetan Plateau, mainly consisting of Paleo-Tethyan I-type granitoids, is a natural laboratory to study the petrogenesis of I-type granitoids and the evolution of continental crust in orogenic belt. In this paper, taking the Paleo-Tethyan granitoids in the EKOB (eastern segment) as the main research object, combined with the typical gabbros, through comprehensive studies of field geology, petrology, mineralogy, geochemistry and geochronology, I make clear the rock types and the temporal-spatial pattern of these Paleo-Tethyan granitoids in the EKOB (eastern segment), figure out their petrogenesis and geodynamic background, and explore the continental growth and evolution mechanism in the EKOB. Combined with the magmatic rocks, stratigraphy and paleontology data from multiple disciplines, I propose a revised Paleo-Tethyan tectonic evolution model for the EKOB.Through the studying of petrography, mineral chemistry, zircon U-Pb geochronology, geochemistry and Sr-Nd-Hf isotope, I summarize the main conclusions as follows:(1) The characteristics of the temporal-spatial distribution of Late Paleozoic to early Mesozoic granitoids have been figure out. The granitic magmatism lasted from270to185Ma, with the peak age of252,240and226Ma. The development of various intrusive rocks is consistent with the characteristics of Andean orogenic belt, i.e.,(gabbro+diorite):(granodiorite+tonalite):(granite+granite)=12:60:28. Spatially, there is more granitoids in the North Kunlun than the South Kunlun, with the ratio of7:3; Temporally, the Triassic granitoids are the most developed ones, which accounted for about48%of the Phanerozoic intrusive rocks, the Permian granitoids accounted for about18%, and the Jurassic granitoids are the least ones; On the whole, the Late Permian granitoids are mostly distributed in the South Kunlun, the Early-Middle Triassic granitoids are mostly distributed in the North Kunlun.(2) The diorite, tonalite and granodiorite are the main ones in the studied area, and mozongranite and K-feldspar granite are the secondary ones. They can be broadly classified into three groups according to zircon U-Pb ages, lithology and the amount of mafic microgranular enclaves (MMEs):the first group covering the time span from Permian to Early Triassic (270～238Ma), is mainly comprised by diorite, tonalite and granodiorite, characterized by numerous MMEs and coeval mafic magmatic rock; the second group emplaced during Middle Triassic (238～230Ma), consisting mainly of silicon-rich granites (granite porphyry, monzogranite and K-feldspar granite), with no or very few MMEs hosted, and contemporaneous mafic magmatism is rare; the third group is late Triassic-early Jurassic granitoids (230-185Ma), with main lithology of (porphyritic or porphyritic) diorite and granodiorite, characterized by numerous MMEs and coeval mafic magmatic rock.(3) Based on elemental and isotopic compositions, the different groups have different rock types and petrogenesis. The first group is metaluminous, high-K calc-alkaline I-type granitoids, and exhibits typical subduction-related geochemical characteristics. They have high Mg#(41～49) and low Rb/Sr (0.02～0.45), with εNd(t)=-6.5～-4.8,(87Sr/86Sr);=0.70807-0.70887and εHf(t)=-9.00～-0.75(average of-3.70). They were derived from partial melting of lower crust mainly composed of Precambrian metabasaltic basement rocks with different degree of involvement of mantle material; The second group is high Rb/Sr, metaluminous to weakly peraluminous, high-K alkali-calcic I-type granitoids, showing characteristics of typical pure crustal-derived granitoids. They have low Mg#(19～39) and high Rb/Sr (0.40～10.83), with εNd(t)=-7.8～-4.8,(87Sr/86Sr);=0.70865～0.71340and εHf(t)=-6.89～-3.90. They were derived from partial melting of Mesoproterozoic metagreywacke source in the lower crust; The third group is metaluminous, high-K calc-alkaline I-type granitoids, and exhibits the typical geochemical characteristics of adakites, e.g. high Sr, low Y and Yb contents. These adaktic rocks could be divided into two period, the former one was derived from partial melting of thickened mafic lower continental crust, without significant crust-mantle interaction (ca.223～225Ma, Sr=444～666ppm, Yb=0.85-1.23ppm, Y=9.7-13.2ppm and εHf(t)=-3.41～ˇ1.10), but the later one experienced significant crust-mantle mixing, characterized by high Mg#adakites with depleted isotopic compositions (ca.209～212Ma, Sr=462～880ppm, Yb=0.73～1.42ppm, Y=9.1～16.2ppm,εHf(t)=1.81～4.04).(4) The Bairiqili gabbros, MMEs and granitoids have similar crystallization ages of ca.250Ma. The gabbros could be divided into two groups:the group A is cumulate-related anorthosite, olivine gabbro and hornblende gabbro, characterized by high Mg#(69-79), low contents of REE and high positive Eu anomaly (ΣEE=1.85～18.46ppm,8Eu=1.2～5.33); the group B is veined fine-grain hornblende gabbro, characterized by moderate Mg#(48-68), high contents of REE and no Eu anomaly (ΣREE=26.16-178.99ppm, average δEu of0.91). These gabbros were mainly derived from low degree partial melting of enriched mantle. The mantle enrichment process was controlled by the slab subduction, in which the upper crust-derived sediments carried by the subducted oceanic crust would mix with these MORB-type mantle in various proportions (1:1-2:3). Partial melting of these mixtures would generate the mafic magma with island arc-like and enriched Sr-Nd-Hf isotopic compositions. Coeval monzogranites were derived from partial melting of Middle Proterozoic lower crust amphibolite, without obvious mantle contribution. Tonalite, granodiorite and MMEs were products of crust-mantle mixing in different proportions, with the mafic endmember of enriched mantle and acid endmember of ancient lower crust.(5) Petrographic and mineralogical data suggest that MME is the symbol of magma mixing. The MMEs have many disequilibrium textures, such as poikilitic plagioclase, pupiled quartz and acicular apatite, indicating the interaction between two unbalanced magmatic systems. Besides, there are two types of plagioclases in MMEs, for example, the MMEs in Bariqili have labradorite (An=49-61) and anorthite (An=80-92). Also, the amphiboles in MMEs have varied composition (Mg#=47-62), which could be divided into Mg-hornblende and Fe-horblende, corresponding to the ones in gabbro and granodiorite, respectively. Elemental geochemistry and model studies show that, the MMEs were formed by magma mixing in different proportions between the enriched mantle magma and granitic magma, characterized by the various mineral assemblages, textures and chemical compositions.(6) Basaltic magma underplating and crust-mantle mixing is the main mechanism for the origin of large-scale I-type granitoids in the EKOB. The underplated basaltic magma provides not only the sufficient heat for lower crust, but also enough material for crust-mantle mixing and the driving force for the emplacement of granitic magma. The mafic magma underplated the overlying lower crust, and induced its partial melting to form felsic magma. The mafic magma first mixed with felsic magma at depth, and then the mixed magma broke up into discrete globules upon after entering the felsic magma chamber, forming enclaves by convective motion, or forceful injection in the host felsic magma. The multiple episodes of mafic dykes developed in the studied area suggest that the magma underplating and crust-mantle mixing ran through the late Paleozoic to early Mesozoic orogeny in the EKOB.(7) Reworking of ancient continental lithosphere is one important mechanism for the continental crust evolution during late Paleozoic to early Mesozoic in the EKOB. This mechanism is significantly different from those in the Gangdese and Central Asian orogenic belts, in which the latter ones show the direct contribution of juvenile mantle. The gabbros in the EKOB show enriched Sr-Nd-Hf isotopic composition, with εNd(t)=-6.0～3.9,(87Sr/86Sr)i=0.71002～0.71148, εHf(t)=-2.37～2.77, and the granitoids have εNd(t) of-7.8～-1.7,(87Sr/86Sr); of0.70804～0.71414, εHf(t) of-9.0～3.04, indicating the continental evolution is the results of different degree of partial melting and mixing of continental lower crust and enriched mantle.(8) Combined with magmatic rocks and other geological data, a revised tectonic evolution model for the Paleo-Tethyan in the EKOB is proposed. The A’nemaqen Paleo-Tethyan ocean is closed in Middle Triassic, and the Late Permian to Early Triassic granitoids and coeval gabbros were formed in subduction-related setting, in which the subducted sediments mixed with the MORB-type mantle in the mantle wedge, and thus took place partial melting to generate the mafic magma under the condition of fluid metasomatism, these mafic magma underplated the ancient lower crustal and induced partial melting, then magma mixing between these mafic and felsic magmas resulted in the generation of numerous subduction-related high-K calc-alkaline I-type granitoids; During the Middle Triassic, the terranes occurred collision, and the former basaltic magma still heated the lower crust, inducing the partial melting of upper part felsic greywacke in the lower crust, with the formation of the second group of high-silicon, high Rb/Sr I-type granites; As early as the Late Triassic, the deep subducted oceanic slab broke off, and the tectonic setting transited into post-collisional extension, deep mantle magma upwelled and underplated to the lower crust, leading the thickened lower crust to melt. Accompanied by the crust-mantle interaction, numerous high-K calc-alkaline, high Mg#adakitic I-type granitoids and coeval OIB-type gabbros generated. |
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