Mineralogy and geochemistry of trachytic rocks from the Lichi Volcanics, Eastern Himalaya: insights into the Kerguelen mantle plume activity in the Eastern Himalayan Region

The Lichi volcanics are a suite of mafic-intermediate-felsic rocks and are considered coeval with the Abor volcanics(~132 Ma) of the Siang window in the Eastern Himalaya. Here, we present the first report of trachytic rocks from the Lichi volcanics, which are exposed in the Ranga valley, along the Kimin-Yazali road section in the Eastern Himalayan Region, Northeast India. The trachytes occur in close association with sandstones of the Gondwana Group of rocks and are characterised based on field, petrographical, and geochemical investigations.These fine-grained trachytes are composed of alkali feldspar, biotite, plagioclase, sodic-amphibole, apatite, illmenite, and titanite. The REE profiles of the evolved trachytic rocks(higher SiO_(2)content) display fractionated trends. The fractionation of accessory mineral phases, like apatite and titanite, was possibly responsible for the strongly fractionated REE patterns of the evolved samples.The trachytic rocks demonstrate high apatite saturation temperatures of 988 ± 14 ℃(1σ, n = 8). The Aluminium Saturation Index(< 1.1) and binary discrimination diagrams of these peralkaline trachytes define their affinity with A-type granitoids. Elemental ratios like Y/Nb, Nb/U,and Ce/Pb signify that the Lichi trachytes are differentiated products of mantle-derived ocean island basalts. Trace elemental discrimination diagrams Th/Yb versus Nb/Yb, Y versus Nb, and Y + Nb versus Rb reflect a within-plate tectonic regime for the trachytes. From the results presented in this work, we infer that the development of rifting events during the breakup of eastern Gondwana due to the onset of Kerguelen plume activity further led to underplating of basic magma in lower crustal levels. These parental basaltic magmas underwent fractionation processes forming differentiated trachyandesites and trachytes.Taking into consideration the similarities recorded between the Lichi volcanics and Abor volcanics, this study supports the idea that Kerguelen plume activities resulted in the emplacement of these volcanics in the Eastern Himalayas.

Trachytic Rock and Associated Fenitization in the Bayan Obo Ore Deposit,Inner Mongolia,China:Evidence for Magmatic-Hydrothermal Mineralization Related to a Carbonatitic Complex

Trachytic rock and its altered rock—fenite—in the Bayan Obo ore district, Inner Mongolia, China, were referred to as slate or feldspar rock before, and identified by the authors for the first time (in 1992). In the paper the mineral assemblages, structures and textures and petrochemical compositions of the rocks, as well as the electron microprobe analysis of feldspars in the rocks are described. The Sm-Nd isochron age of the trachytic rock is 1096 ± 56 Ma, with INd=0.51100±4 (2 s?) and ?Nd(t)= ?4.4 ± 0.7. Alterations of the trachytic rock, including microclinization, riebeckitization, aegirinization and biotitization, and accompanied rare element and REE mineralizations are discussed. Based on the occurrence of the trachytic rock and associated fenitization it is deduced that the Bayan Obo Fe-Nb-REE ore deposit is genetically related to magmatic-hydrothermal activity of an alkali carbonatite complex.

Fractional Crystallization and Crustal Contamination of Doleritic and Trachytic Dykes Crosscutting the Cretaceous Sedimentary Basins from Figuil (North Cameroon) and Léré(South-Western Chad): Geodynamic Implications

Magmatism in the Cretaceous sedimentary basins of the Figuil and Léré regions constitutes one of the fundamental parameters in the reconstruction of the history of the Cretaceous sedimentary basins. The main objective of this paper is to constrain the petrogenetic processes of hypovolcanic rocks and to determine their geodynamic context of emplacement. The petrographic study of mafic hypovolcanic and trachytic rocks was carried out under a polarizing microscope on thin sections. For the geochemical study, the major oxides and some trace elements were analyzed by ICP-AES. Trace and rare earth elements were analyzed by ICP-MS. The dolerites of the Cretaceous sedimentary basins are composed of dykes of amphibole bearing dolerites, biotite and pyroxene bearing dolerite, pyroxene bearing dolerites and trachytes. The dykes are in the order of 20 to 100 m wide by several kilometers long and oriented from N23°E to N90°E. The textures of these rocks are sub-ophitic to intergranular for dolerites and trachytic for evolved rocks (trachytes). The geochemical study shows that the dolerites are basaltic in composition with alkaline to subalkaline character. The sampled dykes have an evolution dominated by fractional with the minor impact of the crustal assimilation characterized by low Rb/Y ratios for dolerites (0.36 - 0.97) and high values of Rb/Y for the Pan-African granitoids’ samples (1.95 - 4.01). The nature of doleritic and trachytic magma sources is supported by their (Tb/Yb)N > 1.9 (1.91 - 3.79) and Dy/Yb > 2 (2.32 - 3.50) ratios of most samples, which suggests melting in a garnet-bearing mantle. Concerning the geodynamic context of the studied rocks, doleritic samples are classified as within-plate tholeiite and volcanic arc basalt, and within-plate alkali basalts.

Apatite Geochemical and Nd Isotopic Insights into Trachyte Petrogenesis in the Tianchi Volcanic Area of Changbai Mountain,NE China

We report the oxide,element geochemistry and Nd isotopic geochemical data of apatite in the middle Pleistocene medium-and fine-grained trachyte in the Tianchi volcanic area(TVA)of Changbai Mountain,discussing the relationship between apatite and the composition of the whole rock.The purpose is to use the apatite geochemical data to constrain the evolutionary process of trachytic magma and the petrogenesis of trachyte in the cone-forming period of the Tianchi volcano.Apatite(Ca_(5)(PO_(4))_(3)(OH,F,Cl))is a common accessory mineral that occurs widely in volcanic rocks in the TVA.The apatites in the trachyte are mainly subhedral-anhedral,having the characteristics of magmatic apatite.In terms of oxide and element geochemistry,they have homogeneous Al_(2)O_(3),SiO_(2),MgO,P_(2)O_(5),K_(2)O,CaO and heterogeneous TiO2,with high F content.They are generally enriched in Th,U and LREEs,depleted in Nb,Ta,Zr,Hf and HFSEs,showing negative Ba,Sr and Ti anomalies,similar to those of the whole-rock host trachytes.The ratios of high(La/Yb)_(N),low δEu(Eu/Eu*),Sr/Y value and ΣREE content in apatite,and the F,Sr,Y,Th/U,La/Sm,and Nd/Tb with ΣREE andδEu anomalies showed a linear correlation,all of those indicating that the host magma has the characteristic of high differentiation.The apatite grains generally having ^(147)Sm/^(144)Nd,^(143)Nd/^(144)Nd ratios and ε_(Nd)(t)values of 0.1072-0.1195,0.5123-0.5126 and -3.49 to -0.10,respectively,are similiar to those of the host rock.The Nd model ages TDM1 are 949-803 Ma in apatite.Combined with theεNd(t)value of the apatite core(-7.06 to-3.49),we conclude that the initial magma of the host trachyte was derived from the partial melting of Proterozoic crustal material and there was an assimilation of wall rocks during its evolution.

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.

CHARACTERISTICS OF ALKALINE VOLCANIC ROCKS OF PLIOCENE IN THE WESTERN PART OF HONGHE FAULT ZONE

A set of pull\|apart basins were formed along the Weixi—Qiaohou right\|lateral slip shear zone of the western part of Honghe fault zone during the period from Miocene to Quaternary.A rock suite of alkaline basalt\|trachyte\|leucite phonolite is distributed in the Pliocene basin developed in the middle and northern parts of the fault. The location of these rocks and the features of the basin indicate the close relationship between the rock suite and the strike\|slipping.. The sedimentological and chronological evidences prove that Dianxi plateau uplifted quickly in the Pliocene. We can get the information about the plutonic process of the uplift of the plateau from this alkaline rock association. There are mainly olivine\|pyroxene\|trachyandesite and biotite\|trachyte in the rock association. The rocks are often of porphyritic texture and block or semi\|directional flowage structure while the matrix is of trachytic or microcrystalline texture. The phanerocrysts are diopsidic augite, rimpylite, biotite and perthite (olivine sometimes can be seen). The matrix are made up of alkaline feldspar microcrystalline (30%～50%), short\|grained diopside (10%～15%), light\|colored volcanic glass (0～15%) and some magnetite, while feldspar microcrystalline in some rocks are arranged directionally. And there are sharp\|edged or round pyroxenite enclaves and hemicrystalline of short grained sinaite, biotite sinaite and felsic breccia in the trachyte, with good demarcation line. The pyroxenite enclaves in the trachyte are of different size, and the size of the biggest ones are 10cm or so with the characteristic of plastic yield flowing. Sinaite hemicrystalline may come from the older intrusions of Pliocene and Eocene epoch.

Petrology and Geochemistry of Igneous Rocks in Zarinkamar Area, NE of Shahrood, Iran

Zarinkamar area is located in northeast of Shahrood, northeast of Iran, between latitudes of 36°37’ - 36°42’ and longitudes of 55°07’ - 55°12’. Different types of igneous rocks have an outcrop on the region. Intrusive igneous rocks include a type of syenite with aegirine pyroxene and the other includes quartz from 5 to 20 percent. Moreover, aegirine syenite, ordinary syenite, and quartz syenite have been injected into Karaj Formation with an age of middle to early Eocene. Also, some parts of trachyte magma have given rise to produce trachyte outcrops after passing the crust and current flows. Studying major and rare elements shows that a magma separation has been caused by separation smelting or a crystal separation. These magmas are alkaline and shoshonites and they have been originated in a continental rift. The magma of Zarinkamar region has been combined with acidic continent crust as it rose to the surface. High amount of Th, and Rb and low amount of Ti, P, Sr, and Ba can prove this hypothesis.

GEOCHEMISTRY OF EARLY CRETACEOUS TRACHYTES OF DONGLINGTAI FORMATION FROM THE XISHAN AREA, BEIJING: CONSTRAINTS ON MELTING OF LOWER MAFIC CRUST

The early Cretaceous trachytes of Donglingtai Formation in Xishan, Beijing are characterized by slight Eu negative anomaly and significant enrichment in LREE, LILE (Ba, K and Sr) and depletion of Nb-Ta-Ti, Th-U. These trachytes have been highly enriched by Sr and Nd isotopic signatures (87Sr/86Sr(t)= 0.70638～ 0.70672, εNd(t) = -16.3～ -15.7), overlapping Sr-Nd isotopic range of late Mesozoic mafic igneous rocks in the region. Taking into account Nb-Ta fractionation and high Zr/Sm ratio for these trachytes, we consider that the trachytes of Donglingtai Formation are derived from the garnet-bearing amphibolite in the lower crust composed of garnet + plagioclase ± amphibole ± pyroxene residual phases. On the basis of the melting experimental results of crustal materials and regional extensional tectonics during late Mesozoic, it is concluded that the thermal input from underplating basaltic melt results in the partial melting of lower continental crust to generate the trachytes of Donglingtai Formation. And the characteristics of high Sr and Ba for these trachytes suggest that part of underplating basalts should take a share in partial melting at least.