Mathiasite-loveringite and priderite in mantle xenoliths from the Alto Paranaíba Igneous Province,Brazil: genesis and constraints on mantle metasomatism

Alkali-bearing Ti oxides were identified in mantle xenoliths enclosed in kimberlite-like rocks from Limeira 1 alkaline intrusion from the Alto Paranaíba Igneous Province, southeastern Brazil. The metasomatic mineral assemblages include mathiasite-loveringite and priderite associated with clinopyroxene, phlogopite, ilmenite and rutile. Mathiasite-loveringite (55–60 wt.% TiO₂; 5.2–6.7 wt.% ZrO₂) occurs in peridotite xenoliths rimming chromite (～50 wt.% Cr₂O₃) and subordinate ilmenite (12–13.4 wt.% MgO) in double reaction rim coronas. Priderite (Ba/(K+Ba)< 0.05) occurs in phlogopite-rich xenoliths as lamellae within Mg-ilmenite (8.4–9.8 wt.% MgO) or as intergrowths in rutile crystals that may be included in sagenitic phlogopite. Mathiasite-loveringite was formed by reaction of peridotite primary minerals with alkaline melts. The priderite was formed by reaction of peridotite minerals with ultrapotassic melts. Disequilibrium textures and chemical zoning of associated minerals suggest that the metasomatic reactions responsible for the formation of the alkali-bearing Ti oxides took place shortly prior the entrainment of the xenoliths in the host magma, and is not connected to old (Proterozoic) mantle enrichment events.     

In-situ zircon U-Pb age and Hf-O isotopic constraints on the origin of the Hasan-Robat A-type granite from Sanandaj–Sirjan zone,Iran: implications for reworking of Cadomian arc igneous rocks

Mineralogy and Petrology - The Lower Permian Hasan-Robat syenogranite occurs as a single pluton and intruded the Upper Carboniferous–Lower Permian sandstones and dolomitic limestones in the...     

Zircon U–Pb geochronological,trace element,and Hf isotopic constraints on the genesis of the Fe and Cu skarn deposits in the Qiman Tagh area,Qinghai Province,Eastern Kunlun Orogen,China

Fe and Cu skarn deposits are very important skarn types worldwide, but it is currently unclear whether the nature of intrusions related to Fe and Cu skarn deposits exerts a key influence on variations in metal associations between Fe and Cu skarn deposits. The Qiman Tagh area of Qinghai Province (QTQP), located in the Eastern Kunlun Orogen (EKO), provides a good opportunity to address this issue. Here, integrating new zircon U–Pb ages, trace elements and Hf isotopes from this study with published data, we constrain the sources of magma associated with Fe and Cu skarn deposits within the QTQP and discuss their role in controlling differences between Fe and Cu skarn deposits.Combined with published data, two discrete suites of the intrusions associated Fe and Cu skarn deposits have been recognized in the QTQP: (1) 245.1 ± 1.5 Ma granodiorite (related to a 245.5 ± 1.6 Ma Cu skarn deposit) has zircon εHf(t) values of −11.9 to −2.1; (2) 235–224 Ma monzonites, quartz monzonites, granodiorite porphyries, monzogranites, and granites associated with 234–225 Ma Fe skarn deposits are characterized by relatively high zircon εHf(t) values (−5.1 to +5.9). The Sr–Nd–Hf isotopic data suggest that the intrusions of Suite 1 and 2 were dominantly derived by partial melting of a Mesoproterozoic juvenile mafic lower crust. Suite 2 intrusions associated with Fe skarn deposits have more mantle components in their magma sources than rocks of Suite 1 that are related to a Cu skarn deposit. Furthermore, zircon εHf(t) values of intrusions associated with Fe and Cu skarn deposits in the QTQP show a negative correlation between mantle components in the magma sources and the contents of Cu and Zn in these deposits. Zircon trace elements indicate that the intrusions associated with Fe skarn deposits are relatively less oxidized than the rock associated with Cu skarn deposit in the QTQP, reflecting a positive correlation between crustal components in the magma sources and oxygen fugacity of the magmas. This indicates that different proportions of mantle and crustal materials in the magma sources may affect oxygen fugacity and Fe contents of the magmas, which possibly leads to the variations in metal associations between Fe and Cu skarn deposits in the QTQP. Zircon U–Pb ages, trace elements and Hf isotopic compositions, combined with geological, geochronological, and geochemical evidence, indicates that having different proportions of mantle components in the magma sources of intrusions associated with Fe and Cu skarn deposits is one of the most critical factors controlling differences in metal association between Fe and Cu skarn deposits.     

Comment on: ‘High-temperature dehydration melting and decompressive P–T path in a granulite complex from the Eastern Ghats India’ by S. Bhattacharya and R. Kar

Thermodynamic and chemographic modelling of complex reaction textures observed in Mg-Al-rich pelitic granulites is an important tool to unravel the P–T evolutionary history of high-grade rocks. In the Eastern Ghats Belt, India, several studies have been carried out on these fascinating aluminous granulites, and the results of these studies have revealed complex P–T–t histories (Dasgupta and Sengupta 1995; Sengupta et al. 1999; Rickers et al. 2001a, 2001b; Gupta et al. 1999; Dobmeier and Simmat 2002; Dobmeier and Raith 2003). In recent communication, Bhattacharya and Kar (2002) reported reaction textures from a suite of Mg-Al granulites from the Paderu area of the Eastern Ghats Belt. Combining the textural relations and thermodynamic calibration of some construed reactions, the authors have put forward a single phase metamorphic evolution of the area along a clockwise pressure–temperature trajectory. Combining the petrological features from the Paderu area with those reported from the Chilka Lake complex, the authors proposed a general tectonic model for the entire Eastern Ghats Belt. Incidentally, the rocks in and around Paderu have been studied in some detail by several other workers (Lal et al. 1987; Mohan et al. 1997; Sengupta et al. 1997). The purpose of this comment is to demonstrate that the conclusions made in the paper are inconsistent with the petrological features described in the text. Further, the thermodynamic treatment used in the paper has serious errors in many places, and hence, is often in complete disagreement with the existing experimental data and theoretical analyses on the Mg-Al-rich assemblages. There are also significant problems arising from the poor quality of the analytical database. Unfortunately, the authors cite only a few published works (mostly their own) ignoring many other relevant studies from this belt (cited above). Our observations are organised according to the sections of the paper.Editorial responsibility: T.L. Grove     

Comment on: “Enriched mantle – Dupal signature in the genesis of the Jurassic Ferrar tholeiites from Prince Albert Mountains (Victoria Land, Antarctica)” by Antonini P. et al. (Contributions to Mineralogy and Petrology 136, 1–19; 1999)

18 O values, the magmas retain initial ⁸⁷Sr/⁸⁶Sr Compositions of greater than 0.709 (e.g. Hoefs et al. 1980). Consistent with this is the lack of a radiogenic Os isotope signature in these rocks (e.g. Molzahn et al. 1996) as would be expected with assimilation of continental crust. Comments here are not concerned with the AFC calculations employed to describe the variations within the suite of samples from the Prince Albert mountains studied by Antonini et al., but rather, the extrapolation of these to more primitive compositions. Antonini et al. have revived the possibility that the unusual chemical features of the least evolved low Ti continental flood basalts of the Ferrar Province reflect a crustal-level interplay between mantle-derived magmas and the continental crust. It is argued here that they have not demonstrated their claim. Received: 7 October 1999 / Accepted: 25 January 2000     

Comment on “Timing and nature of the Xinlin–Xiguitu Ocean: constraints from ophiolitic gabbros in the northern Great Xing’an Range,eastern Central Asian Orogenic Belt” by Feng et al. (2016)

International Journal of Earth Sciences - We disagree the transitional supra-subduction zone model of Feng et al. (Int J Earth Sci (Geol Rundsch) 105:491–505, 2016) for the tectonic setting...