Two clinopyroxene megacrysts, DMP‐2 and DMP‐3, were collected from Cenozoic alkali basalts in the Hannuoba region of China. They were characterised for major and trace element compositions for in situ microanalysis. EPMA and LA‐ICP‐MS analyses indicate homogeneity in the element mass fractions in both clinopyroxene samples. Bulk analyses using various techniques (XRF, ICP‐OES and solution ICP‐MS) also reveal good consistency in their major and trace element data. They, thus, can be used as potential reference materials for elemental in situ microanalysis. Accordingly, element mass fractions are recommended for thirty‐two elements.
Abstract Here we present new occurrences of amphibole in a suite of chromitites, dunites, and harzburgites from the mantle sequence of the Lycian ophiolite in the Tauride Belt, southwest Turkey. The amphibole occurs both as interstitial grains among the major constituent minerals and as inclusions in chromite grains. The interstitial amphibole shows generally decreasing trends in Na2O and Al2O3 contents from the chromitites (0.14–1.54 wt% and 0.04–6.67 wt%, respectively) and the dunites (0.09–2.37 wt%; 0.12–11.9 wt%) to the host harzburgites (<0.61 wt%; 0.02–5.41 wt%). Amphibole inclusions in chromite of the amphibole-bearing harzburgites are poorer in Al2O3 (1.12–8.86 wt%), CaO (8.47–13.2 wt%), and Na2O (b.d.l.–1.38 wt%) than their counterparts in the amphibole-bearing chromitites (Al2O3 = 6.13–10.0 wt%; CaO = 12.1–12.9 wt%; Na2O = 1.11–1.91 wt%). Estimated crystallization temperatures for the interstitial amphibole grains and amphibole inclusions range from 706 to 974 °C, with the higher values in the latter. A comparison of amphibole inclusions in chromite with interstitial grains provides direct evidence for the involvement of water in chromitite formation and the presence of hydrous melt/fluid metasomatism in the peridotites during initial subduction of Neo-Tethyan oceanic lithosphere. The hydrous melts/fluids were released from the chromitites after being collected on chromite surfaces during crystallization. Different fluid/wall rock ratios are thought to have controlled the crystallization and composition of the Lycian amphibole and the extent of modification of the chromite and pyroxene grains in the peridotites. Considering the wide distribution of podiform chromitites in this ophiolite, the link between chromitite formation and melt/fluid metasomatism defined in our study may be applicable to other ophiolites worldwide.
Abstract Potassium (K) isotopes have been used as a tracer of K recycling in the Earth, but K isotope fractionation during magma evolution is poorly constrained. Here, we present K isotope data for a magmatic suite of alkaline silicate-carbonatite affinity. The suite was formed from liquid-liquid immiscibility and subsequent phlogopite fractionation. The K isotopic signatures of different rock types are in the following order: alkaline silicate lavas (δ41K = –0.424 to 0.090‰) > carbonatitic silicate lavas (δ41K = –0.640 to –0.035‰) > carbonatites (δ41K = –0.858 and –0.258‰). Phlogopite phenocrysts in the silicate lavas are isotopically lighter (δ41K = –0.628 to –0.534‰) than the lavas in which they occur (Δ41KPhlogopite-whole rock = –0.502 to –0.109‰). Correlations between δ41K values and chemical proxies of melt immiscibility and phlogopite fractionation indicate that K isotopes are significantly fractionated by both processes at a ~0.6‰ magnitude. Such K isotope variation overlaps the range of δ41K in arc lavas. Compilations of literature data further confirm the critical roles of melt immiscibility and phlogopite fractionation in K isotope variations of high-K lavas (K2O >1 wt%) from post-collision orogenic and intra-continental settings. In comparison, basaltic arc lavas are depleted in K2O (mostly <1 wt%) and lack evidence of significant phlogopite fractionation. The K isotope variations of arc lavas are mainly controlled by their mantle sources, which were metasomatized by melt or fluid released from the subducting slab. Therefore, K recycling and K isotope variation are controlled by distinct mechanisms in different tectonic settings.
Abstract The early-stage subduction records of the Paleo-Asian ocean are poorly preserved in the eastern segment of the Central Asian Orogenic Belt (CAOB), which hinders constraints on the evolution of the whole CAOB. This study presents new age data and zircon Hf-O isotopes as well as bulk-rock geochemistry of the Wuxing mafic-ultramafic complex in the Xingkai massif in northeastern China, which has been identified as Alaskan-type complex with aspects of field occurrence, petrological and mineral assemblages, and mineral chemistry in our recently published work. The results indicate that the complex formed mainly between 517 Ma and 510 Ma with a lithological sequence of Sanying clinopyroxenite (517 Ma), then Sanying gabbro (514 Ma), and Erying hornblendite (513 Ma), and finally Erying hornblende clinopyroxenite (510 Ma). The lithological formation sequence is consistent with the intrusive relations between lithological phases and their irrelevant major element compositions and variable trace element patterns of the bulk rocks. The εHf(t) values of zircon in two samples in this study vary from from +4.45 to +7.61 and from −11.8 to +4.42, respectively and tend to be more depleted with age. These features suggest that the Wuxing complex was a product of long-term arc magmatism and experienced significant ancient crustal assimilation in early-stage magmas and negligible contamination in later ones. The presence of 1222 Ma and 706 Ma inherited zircon grains implies existence of Proterozoic basement in the Xingkai massif and its continental arc setting in Cambrian. The Wuxing complex is the oldest Alaskan-type complex found so far in the entire CAOB and is a good witness of the Paleo-Asian oceanic subduction in the easternmost CAOB. The older age of the Wuxing complex compared to the regional Hongqiling intrusion is also compatible with its Alaskan-type nature and platinum-group element mineralization, which are distinct to the Permian-Triassic Ni-Cu sulfide deposit-hosting maficultramafic intrusions in the southern CAOB.
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)
'/%3e%3cuse xlink:href='%23a' transform='rotate(144 450 300)'/%3e%3cuse xlink:href='%23a' transform='rotate(216 450 300)'/%3e%3cuse xlink:href='%23a' transform='rotate(288 450 300)'/%3e%3c/svg%3e)