Magmatic origin and petrogenesis characterization of syenite rock from Pakkanadu alkaline complex, Southern Granulite Terrain, India: Implication on emplacement and petrogenetic history

The present study mainly focused on understanding the magmatic origin and petrogenesis characterization based on the Petrography, major, trace and Rare Earth Element (REE) signatures in the alkaline syenite from Pakkanadu alkaline carbonatite complex. The alkaline plutons from South Indian granulite terrain are intruded along with Archaean epidote-hornblende gneisses. The study area was carbonatite complexes of Tamil Nadu and is characterized by a group of rock associations Carbonatite-Syenite-Pyroxenite - Dunite. From Harker various patterns Pakkanadu alkaline complex syenite showed increasing trends of SiO2, Al2O3, Na2O + K2O opposite to decreasing order of CaO, Fe2O3, MgO, TiO2, P2O5 and MnO trend, suggest fractionation of clinopyroxene, hornblende, sphene, apatite and oxide minerals and feldspar that ruled the fractionation. The concentration of trace elements enriched in Large Ion lithophile elements (LILE) (Ba, Sr, and Rb) elements and High Field Strength Elements (HFSEs) indicated that the dyke intrusion by differentiation of magma from a mantle source. Rare earth element (REE) distribution of Light rare earth element (LREE) enriched and High rare earth element (HREE) depleted pattern show strongly fractionated pattern with moderate Eu anomalies. Plots of tectonic discrimination diagrams of Pakkanadu samples fall in the field of syn-COLG field to the VAG syn- COLG field. For the first time, this type of study was carried out in the study region in a detailed manner. The present study significantly exposed the petrography, petrogenesis and magmatic origin process in the Pakkanadu alkaline carbonatite complex.

Textures and Chemical Compositions of Nb-Bearing Minerals and Nb Mineralization in the Shuangshan Nepheline Syenite Pluton, East Qinling, China

The Shuangshan alkaline complex located in the Henan province of China is a newly discovered, potentially giant niobium (Nb) deposit. A variety of Nb-bearing minerals including pyrochlore, zircon, and titanite have been identified in this deposit. Distinct textural and chemical differences of pyrochlore and zircon indicate that both have different origins. The magmatic pyrochlore and zircon both have euhedral grains with small sizes. On the other hand, hydrothermal pyrochlore is mainly intergrown on the edge or inside of hydrothermal zircon in the form of an aggregate. Compared with magmatic pyrochlore, the contents of F, Ca, and Na in hydrothermal pyrochlore are obviously high. The texture and composition of hydrothermal pyrochlore and zircon indicate that Ca-bearing hydrothermal alteration resulted in the migration of Nb from Nb-bearing zircon and the reprecipitation of Nb to form aggregate pyrochlore. However, the quantitative calculation shows that the amount of Nb migrated from zircon is very small. Therefore, this study suggests that hydrothermal alteration plays a certain role in the redistribution of Nb, but the enrichment of Nb is limited.

Petrogenesis and U-Pb zircon dating of Chaitma Alkaline Complex from the southern margin of the Central Indian Tectonic Zone: geodynamic implications

In this study, we constrain the petrogenesis and U-Pb zircon age of a newly discovered alkaline complex, christened the Chaitma Alkaline Complex at the southern margin of the Central Indian Tectonic Zone in Central India. The Chaitma Alkaline Complex comprises syenites and gabbro, emplaced coevally and show features consistent with magma mixing. Geochemically, syenites are potassic to ultrapotassic (K2O/Na2O: 0.79-3.42) and contain high Ba (∼800 to 2700 ppm) and Sr (∼1400-3200 ppm). They show enrichment of the light rare earth elements (LREE) relative to the heavy rare earth elements (HREE) (La/Yb: 32-103) and do not display Eu-anomaly. Based on their geochemical signatures such as low MgO (<0.87 wt.%), Ni (8-16 ppm) and Cr (7-44 ppm) contents and prominent Zr-Hf negative anomaly, the syenites are inferred to have been derived by partial melting of a carbonated/metasomatised thickened lower crustal source. The coeval gabbros are undersaturated in silica (41-44 wt.%) with relatively high total alkalis (Na2O+K2O: 3.7-5.1 wt.%), Fe2O3 (17-19 wt.%), P2O5 (3.1-4.9 wt.%), Sr (1600-3400 ppm) and Ba (300-3500 ppm) contents. These have low MgO (<4.8 wt.%), Ni (13-30 ppm) and Cr (18-84 ppm). Their chemistry is interpreted to be the result of interaction with the syenitic magma. These geochemical characters along with high LREE/HREE ratio, negative trough in Nb-Ta, Zr-Hf, Ti, Sr and Rb and positive spike of Pb in spider diagram, and enrichment of LILE over HFSE indicate their derivation from metasomatised subduction modified garnet-peridotite mantle source. Our study indicates that syenites and gabbros of the Chaitma Alkaline Complex were formed from genetically unrelated parental magmas derived from distinct sources. U-Pb dating of zircon yielded magmatic emplacement age of 1626±15 Ma for the syenites. The Chaitma Alkaline Complex was presumably formed during a short period of crustal extension in the midst of a protracted period of continent-continent collision and granulite grade metamorphism (c. 1.71-1.58 Ga) at the southern margin of the Central Indian Tectonic Zone.

Combined In Situ Chemical and Sr Isotopic Compositions and U–Pb Ages of the Mushgai Khudag Alkaline Complex: Implications of Immiscibility, Fractionation, and Alteration

The Mushgai Khudag complex consists of numerous silicate volcanic-plutonic rocks including melanephelinites, theralites, trachytes, shonkinites, and syenites and also hosts numerous dykes and stocks of magnetite-apatite-enriched rocks and carbonatites. It hosts the second largest REE–Fe–P–F–Sr–Ba deposit in Mongolia, with REE mineralization associated with magnetite-apatite-enriched rocks and carbonatites. The bulk rock REE content of these two rock types varies from 21,929 to 70,852 ppm, which is much higher than that of syenites (716 ± 241 ppm). Among these, the altered magnetite-apatite-enriched rocks are characterized by the greatest level of REE enrichment (58,036 ± 13,313 ppm). Magmatic apatite from magnetite-apatite-enriched rocks is commonly euhedral with purple luminescence, and altered apatite displays variable purple to blue luminescence and shows fissures and hollows with deposition of fine-grained monazite aggregates. Most magmatic apatite within syenite is prismatic and displays oscillatory zoning with variable purple to yellow luminescence. Both magmatic and altered apatite from magnetite-apatite-enriched rocks were dated using in situ U–Pb dating and found to have ages of 139.7 ± 2.6 and 138.0 ± 1.3 Ma, respectively, which supports the presence of late Mesozoic alkaline magmatism. In situ 87Sr/86Sr ratios obtained for all types of apatite and calcite within carbonatite show limited variation (0.70572–0.70648), which indicates derivation from a common mantle source. All apatite displays steeply fractionated chondrite-normalized REE trends with significant LREE enrichment (46,066 ± 71,391 ppm) and high (La/Yb)N ratios ranging from 72.7 to 256. REE contents and (La/Yb)N values are highly variable among different apatite groups, even within the same apatite grains. The variable REE contents and patterns recorded by magmatic apatite from the core to the rim can be explained by the occurrence of melt differentiation and accompanying fractional crystallization. The Y/Ho ratios of altered apatite deviate from the chondritic values, which reflects alteration by hydrothermal fluids. Altered apatite contains a high level of REE (63,912 ± 31,785 ppm), which are coupled with increased sulfur and/or silica contents, suggesting that sulfate contributes to the mobility and incorporation of REEs into apatite during alteration. Moreover, altered apatite is characterized by higher Zr/Hf, Nb/Ta, and (La/Yb)N ratios (179 ± 48, 19.4 ± 10.3, 241 ± 40, respectively) and a lack of negative Eu anomalies compared with magmatic apatite. The distinct chemical features combined with consistent Sr isotopes and ages for magmatic and altered apatite suggest that pervasive hydrothermal alterations at Mushgai Khudag are most probably being induced by carbonatite-evolved fluids almost simultaneously after the alkaline magmatism.