Multistage Fractional Crystallization in the Continental Arc Magmatic System: Constraints from the Appinites in Tengchong Block, Southeastern Extension of Tibet

The petrogenesis and evolution process of continental arc magmatism provide insight into discovering the formation and differentiation of continental crust. Therefore, the geochemical, isotopic, and mineralogical analyses were conducted for coeval continental arc igneous rocks in the Tengchong Block to clarify their evolution process in the continental arc magmatic systems. The Middle Triassic appinites in the Tengchong Block, southeastern extension of Tibet, were generated at the subduction setting with zircon U-Pb age of ca. 243 Ma. The Nb/Yb, Zr/Yb, and Ta/Yb ratios along with depleted zircon Hf isotopic compositions indicate a source with an N-MORB affinity for the appinites. However, relatively enriched whole-rock Sr-Nd isotopic compositions with the characteristic of high Sr/Nd, Ba/Th, Th/La, and Th/Nd ratios suggest the source was metasomatized by ~2% subducted sediment-derived fluid. According to the REE ratios modeling, the primary magma of Nabang appinites was due to 5-10% partial melting of such metasomatized mantle source. The appinites are characterized by variable compositions, such as SiO2 contents of 47.82-61.74 wt.% and MgO of 10.61-2.61 wt.%, which resulted from the polybaric and multistage fractional crystallization of a slightly hydrous primary magma in a thick crust. At lower crustal pressures, clinopyroxene was the main fractionating phase, and at middle crustal pressures, amphibole+magnetite were the dominant fractionating phases; predominant plagioclase fractionation occurred at the magma emplacement level. This process could be an effective mechanism to induce the differentiation of continental crust. The fractionation of clinopyroxene and amphibole, accompanied by suppressing plagioclase at lower-middle crustal pressures, induces the high alumina in the evolved melt and forms high-alumina basaltic to andesitic magma.

The Metallogeny of the Lubei Ni–Cu–Co Sulfide Deposit in Eastern Tianshan, NW China: Insights From Petrology and Sr–Nd–Hf Isotopes

The Lubei Ni–Cu–Co deposit situated in western segment of the Huangshan-Jing’erquan mafic–ultramafic rock belt in eastern Tianshan of the Central Asian Orogenic Belt (CAOB). The estimated reserve is approximately 9.11 million tons of ore resources with average grades of 0.82 wt% Ni, 0.52 wt% Cu, and 0.03 wt% Co. The Lubei intrusion is mainly composed of gabbro (phase I), peridotite (phase II), pyroxene peridotite (phase III), olivine pyroxenite (phase IV), and diorite (phase V), which intruded into the early Carboniferous tuffaceous clastic rocks. Zircon Laser Ablation–Inductively Coupled Plasma–Mass Spectrometry (LA–ICP–MS) U–Pb age of the diorite (phase V) from the edge of the intrusion is interpreted as the top-limit metallogenic age, which is consistent with the formation ages of the Huangshan and Xiangshan Ni–Cu deposits in eastern Tianshan. The roughly parallel rare earth element (REE) curves of the Lubei intrusion indicate the magma originated from a homologous source. The slightly enriched large ion lithophile elements (LILE) are compared to high field strength elements (HFSE) with negative Nb and Ta anomalies show that the Lubei intrusion has arc-affiliate geochemical characteristics. The Sr–Nd–Hf isotopes show that the magma was derived from depleted lithospheric mantle, while suffering 4–10% lower crustal contamination with slight contamination of the upper crust. Based on a comprehensive conservation of regional geological, geochemical, and geochronological evidence, the primary magma of the Lubei intrusion was identified that it was derived from the partial melting of metasomatized lithospheric mantle previously modified by subduction events. The Lubei nickel–copper–cobalt sulfide deposit was formed after the primary magma experienced fractional crystallization, crustal contamination, and sulfide segregation in a post-collisional extensional geodynamic setting after the closure of the Kanggur ocean basin in the early Permian.

The Zhidoy massif of ultrabasic-alkaline rocks and carbonatites: its geochemical features, sources and ore potential.

<p> The article describes geological structure of Jidoi massif and its age. The scheme of the massif magmatism has been constructed. Double correlation plots of petrogenic elements of rocks of the massif in which the unified trend of rock structures is observed, are given for verification of correctness of the scheme of magmatism. Spectra of TR and spider diagrams of concentrations of rare elements in rocks of the massif are given. Piroxenites, early rocks of the massif are ores on titanium. Titanium concentrates in three minerals: titanomagnetite, ilmenite and perovskite. The main type of titanium ores is perovskitic type, it is known only in Jidoi massif. Mantle sources of primary magma of the massif is concluded on the basic of geochemistry of isotopes of Sr and Nd. </p>

Conditions of accumulation and fractionation of zirconium and hafnium in alkaline-carbonatite systems

The patterns of the distribution and fractionation of strategic metals (Zr, Hf) in the Kugda intrusion (Polar Siberia) have been studied. The contents of these elements significantly exceed their concentrations in other rocks (Zr 246 ppm, Hf 7.4 ppm). A significant increase in Zr and Hf from early rocks (olivinite and melilite rocks) to later differentiation products, syenites with up to 570 ppm of Zr and 16 ppm of Hf, has been revealed. During the evolution of the Kuga magmatic system, notable fractionation of Zr and Hf occurred. The Zr/Hf ratios in the dike rock, similar in composition to the primary Kugda Massif magma, and the early intrusions are fairly close to that of chondrite (Zr/Hf = 37 [1]), while in the latest phases this ratio increases by almost 5-fold. Our study showed that the distribution coefficient of Hf (Kd = 0.58) in alkaline pyroxenes is noticeably higher than that of Zr (Kd = 0.40). Consequently, fractionation of this mineral leads to an increase in the Zr/Hf ratio in the residual liquids. Another mineral concentrating up to 400 ppm of Zr and up to 1520 ppm of Hf is perovskite, which has a very wide crystallization field in the rocks of the Kugda Massif, especially in the earliest olivinite. The data obtained showed that the Zr/Hf ratio in the perovskite of olivinite varies between 2327, that is, noticeably below both the chondritic and the primary magma values. Early crystallization of perovskite is the main reason for increasing the Zr/Hf ratio in melilitolites (up to 54). Thus, the main process of forming the Kugda Massif was continuous crystallization differentiation, accompanied by a noticeable fractionation of rock-forming and accessory minerals (pyroxene and perovskite).

Oxygen isotope composition of zircons from the Talnakh economic intrusion of the Noril’sk province: first data

This study presents the first results of oxygen isotope analyses (18O) collected on zircons from the Talnakh economic intrusion within the Norilsk province. Zircons from gabbro-diorite, gabbroic rocks of the layered series and plagioclase-bearing wehrlite have similar mantle-like mean 18O values (5,39 0,49; 5,64 0,48 and of 5,28 0,34, respectively), which differ from 18O in zircons from sulfide-bearing melanocratic troctolite with a taxitic texture in the lower part of the intrusion (mean 18O = 6,50 0,98). These new oxygen isotope data support (i) the mantle-derived origin of the primary magma(s), parental to the Talnakh intrusion, and (ii) possible involvement of a crustal component during the formation of sulfide-bearing taxitic-textured rocks.