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This study reports zirconium-in-rutile thermometry data from the Himalayan ultrahigh-pressure eclogites and their retrogressed counterparts. The Zr contents of three textural varieties of rutile i.e. matrix rutile, inclusion phase, and overgrown by ilmenite/titanite from coesite-bearing eclogites, slightly retrogressed eclogites, highly retrogressed eclogites, and amphibolites were analyzed for Zr contents using the electron probe micro-analyser. Majority of the rutile grains exhibited homogeneous chemical compositions in all the aforementioned textural varieties; however, some of grains were relatively heterogeneous. Average temperature values of 729 °C, 727 °C, and 706 °C were obtained for the matrix, inclusion, and overgrown rutiles from UHP eclogites and of 705 °C, 699 °C, and 707 °C for those from HP eclogites, respectively. Slightly retrogressed eclogites yielded similar average T values of 703 °C, 706 °C, and 706 °C, respectively, whereas highly retrogressed eclogites resulted in relatively low average temperatures, ca. 690 °C, 691 °C, and 679 °C, respectively. When compared with these eclogites, lower average T values of 672 °C, 630 °C, and 656 °C were obtained from garnetites, and of 665 °C, 658 °C, and 640 °C from garnet amphibolites, respectively. The relatively homogeneous Zr contents and higher temperature values from the matrix rutile in HP and UHP eclogites suggest peak or near-peak metamorphic crystallization, whereas grains with low Zr contents, yielding low temperature values, were likely to be affected by the late-stage retrogression.

The southern Saharan Metacraton is one of the least geologically constrained regions in the world as bedrock exposures are rare. To the west of Lake Fitri near the communities of Ngoura and Moyto in south-central Chad there are two granitic inliers that form a series of lenticular to ellipsoidal low laying hills. Very little is known about the Lake Fitri inliers and their regional correlation to larger massifs in Chad is undetermined. The granites yielded weighted-mean zircon 206Pb/238U ages of 554 ± 8 Ma and 546 ± 8 Ma indicating they were emplaced ~45 million years after the cessation of arc-related magmatism and the subsequent collision between the Congo-Sao Francisco Craton and the Saharan Metacraton. The rocks have distinct groupings of inherited zircons with ages of ~580 Ma and ~635 Ma suggesting they are at least in part derived by recycling of older crustal rocks. The biotite mineral chemistry, whole rock compositions and petrological modeling indicate the granites were derived by melting of crustal lithologies but the whole rock Nd isotopes (eNd(t) = +1.3–+2.9) are characteristic of a mantle source. The contrasting inheritance-rich nature of the granites with a juvenile Nd isotopic signature is likely due to mixing between magmas derived from juvenile (Neoproterozoic) arc-related crust and asthenospheric magmas. Asthenospheric upwelling was probably a response to post-orogenic lithospheric delamination related to fault movement along the Chad Lineament, a possible extension of the Tchollire-Banyo shear zone that extended to the interior of the Saharan Metacraton. The implications are that lithospheric delamination may not have occurred immediately after collision but rather propagated along a narrow belt that extended well into the central regions of the Saharan Metacraton.

Lava flows that constitute the current edifice of Taranaki volcano, New Zealand, contain a complex crystal cargo including plagioclase, pyroxene, amphibole, oxides, and rarely olivine. Amphibole textures and mineral chemistry indicate that crystals are pargasites entrained at various depths by ascending magmas. The crystals record a complex growth history within a range of temperature and pressure conditions (c. 950–1010 °C at mid- to deep crustal levels). Most amphiboles show the development of distinct reaction rims where the mineral is in contact with the ambient melt. Texturally, the rims are identified as detached, symplectic, granular or coarse. True rim thicknesses vary little (±20%, 1σ, on average) within individual thin sections but show a large variation between samples from different lava flows, from 50 μm. Reaction rim formation on Taranaki amphiboles is attributed to degassing during magma ascent as well as small increases in temperature. Associated with the formation of rims, the amphiboles also show two types of volumetric decomposition, identified as irregular and aligned. The former is indicative of slow reaction of amphibole with melt entrapped in fractures and cleavages during decompression-induced degassing, while the other is indicative of heating-induced breakdown of amphibole triggered by uptake into hot magmas prior to the onset of eruption. The combined evidence indicates that despite the complex and variable growth history of the remobilized crystal cargo, such crystals may be useful in constraining volcanic pre- and syn-eruptive processes and, when appropriate experimental data become available, their rates, which may be of the order of days to weeks.

The initiation of subduction is associated with sequential magmatic responses that lead to the formation of the forearc lithosphere, yet the detailed characteristics of these magmatic activities are not well constrained. Here we use mineral chemistries and bulk-rock trace-element contents of highly-depleted harzburgites from the Guleman and Kizildag ophiolites in Southeast Turkey to examine mantle wedge melting dynamics during subduction initiation. We focus on how different components from the subducting slabs potentially contribute to various stages of magmatism throughout the process. Mineral and bulk-rock compositions of these harzburgites are significantly different from those of abyssal peridotites, suggesting that our harzburgites cannot be explained as residues of anhydrous adiabatic melting and melt-rock interaction at mid-ocean ridges alone. This implies that the petrogenesis of SE Turkey harzburgites involves additional processes and components. Harzburgites with the most depleted heavy-rare earth element (HREE) contents are the ones with the highest abundance of strongly incompatible elements, which can be explained by open-system processes where the peridotites in the mantle wedge experienced melting and infiltration of enriched components simultaneously. Open-system dynamic melting models with continuous flux of sediment-derived melts can account for the observed correlation, but are numerically too low compared to the measured values. Based on the observed fractionation between Zr, Hf, and elements with similar incompatibility (middle REEs), we hypothesized the involvement of amphibolite-derived melt and modeled its numerical trace-element contents. Binary mixing between this hypothetical melt and residues of the former open-system model can coherently account for the majority of the obtained trace-element data. This indicates that magmatic events during subduction initiation likely involve multiple components and occur in multiple stages, and that melt-mantle interaction plays a significant role in oceanic forearc lithosphere formation. Based on our model, we suggest the high (Zr/MREE)N signatures in some boninites and depleted harzburgites found in modern forearcs and ophiolites could be inherited from amphibolite-derived melts. Moreover, the existence of slab melts agrees with current constraints on the reconstructed geothermal gradients during subduction initiation based on the petrology and geochemistry of metamorphic soles.

This study reports zircon LA-ICPMS U–Pb ages of 50 igneous rock samples from the Urumieh–Dokhtar magmatic arc (UDMA) and Sanandaj–Sirjan structural zone (SSZ) in Iran. These results, together with literatures and our unpublished age data, better delineate the magmatic evolution related to the Neotethyan subduction and subsequent Zagros orogeny that resulted from the Arabia–Eurasia collision. Subduction-related magmatism was active during Jurassic time, as evidenced by the presence of widespread I-type granitoids from the Middle to Late Jurassic (176–144 Ma) in the SSZ. After a protracted magmatic quiescence in the Early Cretaceous, igneous activity renewed inland in the UDMA from which we identify Late Cretaceous granitoids (81–72 Ma) in Jiroft and Bazman areas, the southeastern segment of the UDMA. The UDMA volcanism was most active and widespread during the Eocene and Oligocene (55–25 Ma), much longer lasting than previously thought as just an Eocene pulse. Such a prolonged igneous “flare-up” event in the UDMA can be correlated to Armenia where coeval calc-alkaline rocks are common. The UDMA magmatism ceased progressively from northwest to southeast, with magmatic activities ending the Early Miocene (ca. 22 Ma) in Meghri, the Middle Miocene (ca. 16 Ma) in Kashan and the Late Miocene (ca. 10–6 Ma) in Anar, respectively. The southeastward magmatic cessation is consistent with the notion of oblique and diachronous collision between Arabia and Eurasia. Post-collisional volcanism started ca. 11 Ma in Saray, east off the Urumieh Lake, which, along with later eruptions in Sahand (6.5–4.2 Ma) and Sabalan (≤ 0.4 Ma) volcanoes, forms a compositionally unique component of the vast volcanic field covering much of the Lesser Caucasus, NW Iran and eastern Anatolia regions.

Zircons from two Myanmar jadeitite samples were separated for texture, mineral inclusion, U–Pb dating and trace element composition analyses. Three types of zircons, with respect to U–Pb isotope system, were recognized. Type I zircons are inherited ones, yielding an igneous protolith age of 160 ± 1 Ma; Type II zircons are metasomatic/hydrothermal ones, giving a (minimum) jadeitite formation age of 77 ± 3 Ma; and Type III zircons are incompletely recrystallized ones, with non-coherent and geologically meaningless ages from 153 to 105 Ma. These Myanmar jadeitites would therefore have formed through whole-sale metasomatic replacement processes. Compared with Type I zircons, Type II zircons show typical metasomatic/hydrothermal geochemical signatures, with low Th/U ratio ( The Myanmar jadeitites, based on the present study, might have formed during the Late Cretaceous subduction before the beginning of India–Asia continental collision at Paleocene. Previously proposed Late Jurassic ages for Myanmar jadeitites are suggested as results rooted on data retrieved from incompletely recrystallized inherited zircons.

a Institute of Earth Sciences, Academia Sinica, Taipei, 11529, Taiwan, ROC b Graduate School of Engineering and Resource Science, Akita University, Japan c Department of Applied Arts, Fu-jen Catholic University, Hsinchuang, Taiwan, ROC d Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada e Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, USA