Petrogenesis of the Late Eocene to Early Oligocene Yao'an Shoshonitic Complex,Southeastern Tibet:Partial Melting of an Ancient Continental Lithospheric Mantle beneath the Yangtze Block

Cenozoic potassic-ultrapotassic igneous rocks are widespread in the southeastern Tibetan Plateau.Their petrogenesis and magmatic processes remain subject to debate in spite of numerous publications.Almost all of the Cenozoic extrusive and intrusive rocks in the Yao’an area,western Yunnan Province,SW China,are geochemically shoshonitic,collectively termed here the Yao’an Shoshonitic Complex(YSC).The YSC is located in the(south)easternmost part of the ENE-WSW-trending,~550 km-long and~250 km-wide Cenozoic magmatic zone;the latter separates the orthogonal and oblique collision belts of the India-Eurasia collision orogen.Previously published geochronological and thermochronological data revealed that the rocks of the YSC were emplaced over a short timespan of 34-32 Ma.This and our new data suggest that the primary magma of the YSC likely was formed by partial melting of ancient continental lithospheric mantle beneath the Yangtze Block.This part of the continental lithospheric mantle had likely not been modified by any oceanic subduction.Fractionation crystallization of an Mg-and Ca-bearing mineral and TiFe oxides during the magmatic evolution probably account for the variable lithologies of the YSC.

Association of Late Paleozoic Adakitic Rocks and Shoshonitic Volcanic Rocks in the Western Tianshan, China

The late Paleozoic adakitic rocks are closely associated with the shoshonitic volcanic rocks in the western Tianshan Mountains, China, both spatially and temporally. The magmatic rocks were formed during the period from the middle to the late Permian with isotopic ages of 248-268 Ma. The 87Sr/86Sr initial ratios of the rocks are low in a narrow variation range (-0.7050). The 143Nd/144Nd initial ratios are high (-0.51240) with positive εND(t) values (+1.28-+4.92). In the εNd(t)-(87Sr/86Sr)i diagram they fall in the first quadrant. The association of the shoshonitic and adakitic rocks can be interpreted by a two-stage model: the shoshonitic volcanic rocks were formed through long-term fractional crystallization of underplated basaltic magma, while the following partial melting of the residual phases formed the adakitic rocks.

Petrogenesis of shoshonitic granitoids,eastern India:Implications for the late Grenvillian post-collisional magmatism

Many elongated, lenticular plutons of porphyritic granitoids are distributed mainly near the southern and northern margin of the Chhotanagpur Gneissic Complex （CGC） which belongs to the EW to ENE-WSW tending 1500 km long Proterozoic orogenic belt amalgamat ng the North and South Indian cratonic blocks. The late Grenvillian （1071 ±64 Ma） Raghunathpur porphyritic granitoid gneiss （PGG） batholith comprising alkali feldspar granite, granite, granodiorite, tonalite, quartz syenite and quartz monzonite intruded into the granitoid gneisses of southeastern part of CGC in the Purulia district, West Bengal and is aligned with ENE-WSW trending North Purulia sr^ear zone, Mineral chemistry, geochemistry, physical condition of crystallization and petrogenetic model of Raghunathpur PGG have been discussed for the first time. The petrographic and geochemical features （including major and trace- elements, mineral chemistry and S7Sr/S6Sr ratio） suggest these granitoids to be classified as the shosh- onitic type. Raghunathpur batholith was emplaced at around 800 ~C and at 6 kbar pressure tectonic discrimination diagrams reveal a post-collision tectonic setting while structural studies reveal its emplacement in the extensional fissure of North Purulia shear zone. l＇he Raghunathpur granitoid is compared with some similar granitoids of Europe and China to draw its petrogenetic model. Hybridi- zation of mantle-generated enriched mafic magma and crustal magma at lower crust and later fractional crystallization is proposed for the petrogenesis of this PGG. Mafic magma generated in a post-collisional extension possibly because of delamination of subducting slab. Raghunathpur batholith had emplaced in the CGC during the final amalgamation （~ 1.0 Ga） of the North and South Indian cratonic blocks. Granitoid magma, after its generation at depth, was transported to its present level along megadyke channel, ways within shear zones.

GEOCHEMISTRY OF THE PLIOCENE SHOSHONITIC ROCKS FROM OIYUG BASIN, CENTRAL TIBET

Cenozoic volcanism has been well studied in northern, eastern and southwestern Tibet (Coulon et al., 1986; Arnund et al., 1992; Turner et al., 1993, 1996; Deng, 1978, 1998; Miller et al., 1999, Wang et al, 2000). But the data of the Cenozoic post\|collisional volcanism in central and southern Tibet is limited (Coulon et al., 1986; Turner et al., 1996; Zhang, 1998). These potassic and ultrapotassic intrusive and extrusive rocks are regarded to be a key clue for the deep lithospheric process of the plateau after Indo\|Asian collision. Present here is the preliminary results of the rocks from Oiyug (Wuyu) basin, about 150km northeast to Shigatse. Major and trace elements of the rocks are studied and compared with the ultrapotassic and potassic (shoshonitic) rocks exposed in Lhasa block and Qiangtang terrane.(1) Geologic setting. The magmatic rocks studied are Gazacun formation of the lower part of the Pliocene Oiyug group (N 2 oy ). The cross\|section is situated in Gazacun village of Namling. Gazacun formation is underlain unconformable by the andesitic rocks of Linzizong Group [(K 2—E 2) ln ], and covered by the coarse\|grain layered sandstone and conglomerate of Zongdangcun formation. The Gazacun formation consists of mediate\|acidic volcanics, granite\|porphyry, coal\|bearing clastics, plants and sporo\|pollen.

Geochemistry and Emplacement of Post-collisional Shoshonitic Dyke Swarms, NW of Iran

About 300 mafic dykes are intruded Eocene volcanic and pyroclastic rocks,north east of Meshkinshahr city in the NW of Iran.Most of dyke swarms display NNE-SSE trend.Their composition varies from basalt to tephrite and

Geological Implications for the Deep-Derived Mafic Enclaves from Cenozoic Shoshonitic Rocks in Jianchuan-Dali Area, West Yunnan

There exist intermediate to intermediate-acid shoshonitic rocks dated at 26.3- 36.7Ma in the Early Tertiary extensional basins along the NE flank of the Dali-Jianchuan section of the Honghe-Ailaoshan left-lateral strike-slip ductile shear zone, where some high- and medium-high grade metamorphosed mafic enclaves have been found. According to the P-T conditions and mineral assemblage characteristics, the enclaves are grouped into three types: type-Ⅰ, garnet diopsidite, formed at lower crust or in the transitional section between the mantle and the crust with the metamorphic condition P=(10.47-11.51)×108 Pa and T=771-932℃; type-Ⅱ, garnet- and diopside-bearing amphibolite, formed at the depth of middle to lower crust with the metamorphic condition: P=(6-10) ×108 Pa and T=780-830℃; type-Ⅲ, metamorphosed gabbro, formed at relatively higher levels than the above 2 types with lower metamorphic condition. Type-Ⅰand type-Ⅱenclaves had experienced a decompressional retrograde metamorphism caused by tectonic elevation associated with the metasomatism of SiO 2, Na 2O-rich fluid phase with the new equilibrated temperature at T=761-778℃. Geochemical evidence indicates that both deep-derived mafic enclaves and host rocks have similar EM Ⅱ properties, which are related to the involvement of subducted materials. The sustaining subduction, compression and strike-slip ductile shearing between the India plate and the Yangtze craton are the main constraints on the subcontinental mantle properties, interaction between crust and mantle, and the formation and evolution of alkali-rich magma.

Alkali feldspar syenites with shoshonitic affinities from Chhotaudepur area: Implication for mantle metasomatism in the Deccan large igneous province

Two petrologically distinct alkali feldspar syenite bodies （AFS-1 and AFS-2） from Chhotaudepur area, Deccan Large Igneous Province are reported in the present work. AFS-1 is characterized by hypidio-morphic texture and consists of feldspar （Or55Ab43 to Or25Ab71）, ferro-pargasite/ferro-pargasite horn-blende, hastingsite, pyroxene （Wo47, En5, Fs46）, magnetite and biotite. AFS-2 exhibits panidiomorphic texture with euhedral pyroxene （Wo47-50, En22-39, Fs12e31） set in a groundmass matrix of alkali feldspar （Or99Ab0.77 to Or1.33Ab98）, titanite and magnetite. In comparison to AFS-1, higher elemental concentra-tions of Ba, Sr and PREE are observed in AFS-2. The average peralkaline index of the alkali feldspar syenites is w1 indicating their alkaline nature. Variation discrimination diagrams involving major and trace elements and their ratios demonstrate that these alkali feldspar syenites have a shoshonite affinity but emplaced in a within-plate and rifting environment. No evidence of crustal contamination is perceptible in the multi-element primitive mantle normalized diagram as well as in terms of trace elemental ratios. The enrichment of incompatible elements in the alkali feldspar syenites suggests the involvement of mantle metasomatism in their genesis.

Shoshonitic intrusive suite in SE Guangxi: Petrology and geochemistry

A NE-direction Mesozoic shoshonitic intrusive s黫te in SE Guangxi has been identified in terms of geological, petrological and geochemical investigations. The shoshonitic intrusives are characterized by enrichment of LILE, HFSE and LREE and no obvious Nb-Ta depletion, similar to those potassic rocks formed in within-plate and rifting environments. Unlike most shoshonitic rocks forming in arc settings, the SE Guangxi shoshonitic intrusives were likely generated during regional lithosphere extension induced by upwelling of asthenosphere mantle.

Isotopic characteristics of shoshonitic rocks in eastern Qinghai-Tibet Plateau: Petrogenesis and its tectonic implication

The Cenozoic magmatic rocks of shoshonitic series in the easternQinghai-Tibet Plateau include potassic alkaline plutonic rocks, volcanic rocks, lamprophyres and acidic porphyries. Analytical results show that these different lithological rocks are extremely similar in Sr, Nd and Pb isotopic compositions with the range of 0.705 187-0.707 254 for 87Sr/86Sr, 0.512 305-0.512 630 for 143Nd/144Nd, 18.53-18.97 for 206Pb/204Pb, 15.51-15.72 for 207Pb/204Pb and 38.38-39.24 for 208Pb/204Pb. They are isotopically similar to the EMII end-member. This indicates that mantle metasomatism must have taken place in their source region. The formation of these particular rocks is related to crustal thinning and mantle upwelling in a large-scale strike-slip and pull-apart fault zone at about 40 Ma in northern and eastern Qinghai-Tibet Plateau.

The shoshonitic volcanic rocks at Hongliuxia:Pulses of the Altyn Tagh fault in Cretaceous?

Two periods of magma activities, i.e. 106-112 Ma shoshonitic eruption and 82 Ma trachytic intrusion, are recognized at the intersection of the Altyn Tagh fault and the northern marginal fault of the Qilian terrane, northeast of the Tibetan Plateau, which were previously considered to be Quaternary magma activities. The two rock-types have significant differences in mineral assemblage and chemical composition, and formed during two different pulses of magma activity. The Cretaceous magma activities and deformation in or near the Altyn Tagh fault zone suggest a possibly tectonic reactivation after the intense activity of the fault during Indusinian.

Zircon U-Pb Geochronology and Geochemistry of the Early Cretaceous Volcanic Rocks from the Manitu Formation in the Hongol Area, Northeastern Inner Mongolia

We undertook zircon U-Pb dating and geochemical analyses of volcanic rocks from the Manitu Formation in the Hongol area, northeastern Inner Mongolia, to determine their age, petrogenesis and sources, which are important for understanding the Late Mesozoic tectonic evolution of the Great Xing＇an Range. The volcanic rocks of the Manitu Formation from the Hongol area consist primarily of trachyandesite, based on their chemical compositions. The zircons from two of these trachyandesites are euhedral-subhedral in shape, display clear oscillatory growth zoning and have high Th/U ratios （0.31- 1.15）, indicating a magmatic origin. The results of LA-ICP-MS zircon U-Pb dating indicate that the volcanic rocks from the Manitu Formation in the Hongol area formed during the early Early Cretaceous with ages of 138.9-140.5 Ma. The volcanic rocks are high in alkali （Na2O ＋ K2O = 6.22-8.26 wt%）, potassium （K2O = 2.49-4.58 wt%） and aluminium （Al2O3 = 14.27-15.88 wt%）, whereas they are low in iron （total Fe2O3 = 3.76-6.53 wt%） and titanium （TiO2 = 1.02-1.61 wt%）. These volcanic rocks are obviously enriched in large ion lithophile elements, such as Rb, Ba, Th and U, and light rare earth elements, and are depleted in high field strength elements, such as Nb, Ta and Ti with pronounced negative anomalies. Their Sr-Nd-Pb isotopic compositions show positive εNd（t） （＋0.16%o t0＋1.64%o） and low TDM（t） （694--767 Ma）. The geochemical characteristics of these volcanic rocks suggest that they belong to a shoshonitic series and were likely generated from the partial melting of an enriched lithospheric mantle that was metasomatised by fluids released from a subducted slab during the closure of the Mongol- Okhotsk Ocean. Elemental and isotopic features reveal that fractional crystallization with the removal of ferromagnesian minerals, plagioclase, ilmenite, magnetite and apatite played an important role during the evolution of the magma. These shoshonitic rocks were produced by the partial melting of the enriched lithospheric mantle in an extensional regime, which resulted from the gravitational collapse following the final closure of the Mongoi-Okhotsk Ocean in the Middle-Late Jurassic.

Metallogeny of Shoshonite-Hosted Copper-Gold Deposits in Middle-South Parts of Tancheng-Lujiang Deep Fault Zone and Its Vicinity, Eastern China

The middle south parts of Tancheng Lujiang deep fault zone and its vicinity are an important locality of Cu Au deposits related to Mesozoic volcanic subvolcanic magmatism in eastern China. According to their metallogenic features and ore forming conditions, copper gold deposits in this district are ascribed to two groups: the epithermal group which can be further divided into tellurium gold type, quartz adularia type and quartz manganoansiderite type; the magmatic hydrothermal group which includes porphyry Cu Au deposit, breccia pipe porphyry type Au Cu deposit and skarn type Au Cu deposit. In this paper, characteristics of six typical shoshonite hosted Cu Au deposits are outlined. Additionally, the factors that control the metallogenesis and distribution of these Cu Au deposits are discussed preliminarily.

Characteristics of Volcanic Rocks in the Shoshonite Province, Eastern China, and Their Metallogenesis

The shoshonite province in eastern China is characterized by extensive distribution (ca. 80000 km2) of Late Jurassic-Early Cretaceous (165-100 Ma) shoshonite series with subordinate high-K calc-alkali series. It was formed in a dominantly tensile stress field. In comparison with their analogues in island arcs and active continental margins in other countries, the volcanic rocks in the shoshonite province have their specific characteristics in petrology, mineralogy and geochemistry as well as related mineralization association, which are the comprehensive reflection of the special composition and structure of the mantle and crust of the province and the special Mesozoic regional tectonic setting.

TWO TYPES OF XENOLITH—MANTLE XENOLITH AND CRUSTAL XENOLITH FOUND IN THE CENOZOIC VOLCANIC ROCKS FROM HOH XIL, NORTHERN TIBET PLATEAU

It is known that large amounts of Cenozoic high potassium volcanic rocks occur on the Tibetan plateau. The question is where do those high potassium magmas come from? Since the plateau is being compressed by subduction and collision from the surrounding continents,it is a puzzle for us what is the formational environment and tectonic setting of these volcanic rocks? In particular,what is the relationship between these special rocks and formation and uplift of the plateau? We recently carried out an investigation on these Cenozoic volcanic rocks in Hoh Xil,northern Tibetan plateau,including volcanic rocks in the Jingyuhu,Xiongyingtai and Shuangquanzi areas. In the region,some older lava flows occur as thick sheets with a flat surface and weathering crust; while some relatively younger lavas remain a relict volcanic cone or vent. Most of the relict cones are small (<100m in diameter) and some are over hundred meters,distributed along faults,particular strike\|slip fault,e.g.,East Kunlun strike slip fault. Relatively,intermediate\|acid volcanic rocks are prominent,and basic\|intermediate and acid are subordinate. Petrological and geochemical results show that these volcanic rocks have many common affinities,characterized by high contents of w (K 2O)+ w (Na 2O) (about 8%),and w (K\-2O)/ w (Na\-2O)>1 or close to 1,and mostly belonged to shoshonite\|trachyte association and some to ultra\|potassic rocks. The K\|Ar dating of whole rock and mica shows that the volcanism lasts from middle Miocene (15 47Ma) to Pleistocene (0 69Ma); they are 0 69Ma,13 77Ma and 15 47Ma in Jingyuhu; 11 05Ma,12 22Ma and 12 83Ma in Xiongyingai,; 1 58Ma,2 24Ma,5 23Ma,5 85Ma,8 20Ma,8 41Ma and 10 67Ma in Shuangquanzi.Two types of xenolith and xenocryst were found in Jingyuhu and Xiongyingtai,i.e..,crustal xenolith and mantle xenocryst.

Post-collisional magmatism in Wuyu basin,central Tibet:evidence for recycling of subducted Tethyan oceanic crust

The trachyte and basaltic trachyte and intruded granite-porphyry of Gazacun formation of Wuyu Group in central Tibet are Neogene shoshonitic rocks. They are rich in LREE, with a weak to significant Eu negative anomalies. The enriched Rb, Th, U, K, negative HFS elements Nb, Ta, Ti and P, and Sr, Nd and Pb isotope geochemistry suggest that the volcanic rocks of Wuyu Group originated from the partial melting of lower crust of the Gangdese belt, with the involvement of the Tethyan oceanic crust. It implies that the north-subducted Tethys ocean crust have arrived to the lower crust of Gangdese belt and recycled in the Neogene magmatism.

Alkali-rich igneous rocks and related Au and Cu large and superlarge deposits in China

The alkali-rich igneous rocks in China occur as fifteen linear distributed belts of each extending several hundreds to several thousands kilometers in length. These include most types of alkali-rich igneous rock categories discovered worldwide. The related Au, Cu large and superlarge deposits or metallogenic focus-areas include Dongping, Guilaizhuang, Yulong, etc. Direct and indirect genetic links have been found between alkali-rich igneous rocks and Au, Cu mineralization. The petrogenesis and metallogenesis of the alkali-rich igneous rocks are mainly controlled by (1) mantle enrichment, (2) strong interaction between mantle and crust, (3) lower contens of sulfur, high and high contents of volatiles, (4) the significant turn of regional tectonic framework from compress to extension and (5) the strong change of regional lithosphere structure.

Petrogenesis of the Mesozoic intrusive rocks in the Tongling area, Anhui Province, China and their constraint on geodynamic process

Petrology, element and isotopic geochemistry of the Mesozoic intrusive rocks in the Tongling area were systematically investigated in this study. The intrusive rocks can be divided into two groups, one contains shoshonitic rocks with SiO2≤55%, the other consists mainly of high-potassic calc-alkaline rocks with SiO2>55%. The shoshonitic rocks (SiO2≤55%) were generated by the fractional crystallization of the primary basaltic magma sourced from an enriched mantle, then the evolved basaltic magma likely experienced low-degree contamination with the lower crust materials when they ascended. On the other hand, although the intrusive rocks with SiO2>55% show most elemental geochemical characteristics similar to an adakite, such as high Na2O, Al2O3, Sr contents, high Sr/Y and La/Yb ratios, they have isotopic compositions much different from an adakite, such as low e Nd(t) (-9.16—16.55) and high (87Sr/86Sr)i (0.7068—0.7105), and some of them show relatively high Y and Yb contents than those of an adakite. We propose that the intrusive rocks with SiO2>55% were most probably produced by mixing of the mantle-derived basaltic magma and adakite-like magma derived from the melting of basaltic lower crust that was heated by the underplating mantle-derived shoshonitic magmas. The delamination of lower crust likely took place after or during the formation of these adakite-like rocks in the Tongling area.