Plagioclase Stability Field Research Articles

Underplating-induced trans-crustal melting and maturation of Neoarchean continental crust in the North China Craton

A global change in granitoid compositions from early predominantly sodic tonalite-trondhjemite-granodiorites (TTGs) to later TTGs and more potassic granites occurred during the late Archean, coupled with a major period of crustal maturation. However, the detailed relationship between granitoid chemical evolution and the maturing crustal process remains enigmatic. Successive granitoid magmatism including late Neoarchean TTGs and high-K granites occurred in the Western Shandong Province granite-greenstone belt (WSP) of the North China Craton and thus preserves crucial clues of the crustal maturation process. In this study, petrology, whole-rock geochemistry, and zircon U-Pb and Lu-Hf isotopes are reported for the late Neoarchean TTG gneisses, monzogranites, and minor metabasaltic to andesitic rocks from the WSP. The ca. 2560−2540 Ma TTG gneisses show low MgO, K2O/Na2O, but high (La/Yb)N, Sr/Y, and absence of Eu anomalies, indicating their derivation from partial melting of the thickened lower mafic crust. The ca. 2530−2500 Ma monzogranites are characterized by systematically high SiO2 and K2O/Na2O, but low MgO and Sr/Y, and moderately negative Eu anomalies, revealing they were formed by intracrustal reworking of local TTGs and sedimentary rocks in the middle to upper crust. Geochemical variations of these crustal-derived granitoids suggest that they were formed by melting at gradually higher crustal levels with the melt zone moved gradationally from the eclogite stability field into the plagioclase stability field. The ca. 2530−2500 Ma calc-alkaline metabasaltic to andesitic rocks sourced from metasomatized mantle outline roles of mantle-derived magma underplating in contributions of heating and trans-crustal melting magmatism. The long-term melting processes facilitated the upward movement of volatiles and heat-producing elements from deep to shallow crustal levels, and introduced K-enriched monzogranites into the upper crust, leaving a refractory, strengthening, and tectonically stable lower crust. Secular compositional evolution of crustal-derived granitoids reveals that continuous crustal reworking drove lithosphere differentiation and paved the way for the maturation of the Archean continental crust.

New approach to constraining Cr-mineralization and forearc processes: U-Pb dating and geochemistry of titanites in chromitites of the Dun Mountain ophiolite belt (New Zealand)

Ophiolites provide important insights for understanding of subduction zones and forearc mantle-wedge geodynamics. Specifically, within the mantle section, podiform chromitites may record processes associated with development of highly depleted supra-subduction ophiolites, chromium mineralization and subsequent metasomatism. This study explores the first ever U-Pb isotope and trace-element data obtained from titanites in chromitites of the Dun Mountain ophiolite belt in New Zealand. Titanite is unusual in such geochemically depleted rocks and a low recovery rate attests to its rarity (ca. 190 grains from 100 kg of material from two chromitite samples). Results of U-Pb geochronology and geochemistry for the titanites constrain both the mid-Permian timing of chromitite crystallization (∼283 Ma) and later metasomatism (∼272 Ma). Older titanite grains are depleted in HFSE and have flat REE chondrite-normalized patterns reflecting a high-degree of partial melting in depths greater than the plagioclase stability field (>15 km). Younger titanites show enrichment in HFSE, depletion in LREE and negative Eu anomalies, revealing melt extraction at shallower depths (<15 km) that likely record refertilization of the mantle-wedge. Results highlight the potential of titanites in the acquisition of direct geochronological and geochemical constraints from rocks commonly devoid of minerals suitable for U-Pb dating.

Rheology of hydrated plagioclase at lower crustal conditions: Cataclasis, creep and transformational plasticity

Plagioclase feldspar is one of the main rock-forming minerals of the lower continental crust. Assessing its rheology is therefore of particular importance for understanding the mechanical processes acting in convergence zones, where pressure and strain rate increases are often associated with metamorphic reactions and fluid fluxes. In this prospect, we performed axial shortening experiments on “as-is” (∼0.05 wt% H2O) and water-added (∼0.29 wt% H2O) natural plagioclase samples in a Griggs-type apparatus equipped with an acoustic emission monitoring system. Samples were deformed at ∼ 10−6 and ∼ 10−5 s−1, 700–800 °C, and 1.0 GPa in the plagioclase stability field. Experiments have also been performed at 1.5 GPa in order to study the rheological impact of the breakdown reaction Ca-rich plagioclase + water → zoisite + kyanite + quartz + Na-rich plagioclase. Mechanical data indicate that at the conditions studied, the rheology of plagioclase is strongly affected by water content. Microstructural observations confirm that under dry conditions, plagioclase aggregates deform by cataclastic flow. Under wet conditions, dislocation creep appears to be the dominant deformation mechanism, and plagioclase rheology is best described by the following equation, where strain rate, in s−1, is expressed as:ε·=Aσ3.0exp(−(168kJ/mol)/R/T), where σ is the stress in MPa, R is the gas constant, T is the temperature in K, and the pre-exponential term A=10−3.9MPa−3.0s−1. At 1.5 GPa, the reaction is faster under wet conditions but the rheology observed remains identical to that of wet plagioclase in its stability field. In the low water content experiments, plagioclase breakdown induces transient weakening due to fast zoisite nucleation and significant reaction volume change.

Ascent-driven differentiation: a mechanism to keep arc magmas metaluminous?

Arc magmatism is fundamental to the generation of new continental or island arc crust. However, the mechanisms that add to the chemical complexity of natural calc-alkaline magmas ranging from basaltic to rhyolitic compositions are debated. Differentiation mechanisms currently discussed include magma mixing, assimilation, crustal melting, or (fractional) crystallisation. In this contribution, the differentiation of arc magmas by decompression-driven crystallisation is investigated. We performed a set of equilibrium crystallisation experiments at variable crustal pressures (200–800 MPa) on a hydrous high-Al basalt (3.5 wt.% of H2O in the starting material) with run temperatures varying from near-liquidus conditions (1110 °C) to 900 °C. Oxygen fugacity was buffered at moderately oxidising conditions close to the NNO equilibrium. Combining these novel experiments with previous polybaric fractional crystallisation experiments (Marxer et al., Contrib Mineral Petrol 177:3, 2022) we demonstrate the effects of pressure on the crystallisation behaviour of calc-alkaline magmas with respect to liquid and cumulate lines of descent, mineral chemistry, and phase proportions. Decompression shifts the olivine-clinopyroxene cotectic curve towards melt compositions with higher normative clinopyroxene and enlarges the stability field of plagioclase. This exerts a key control on the alumina saturation index of residual liquids. We argue that near-adiabatic (or near-isothermal) decompression accompanied by dissolution of clinopyroxene entrained during residual melt extraction in the lower crust keeps arc magmas metaluminous during crystallisation-driven differentiation thereby closely reproducing the compositional spread observed for natural arc rocks.

Post-collisional magmatism associated with the final closure of the Rushan-Pshart Meso-Tethys Ocean in Pamir, Tajikistan: Inference from Cretaceous igneous rocks of the Pshart accretionary complex

The Pamir orogen was formed by the subducted accretion and amalgamation of Cimmerian terranes from the northern margin of Gondwana with the southern margin of Eurasia. The Mesozoic magmatic rocks are widespread in Pamir and record the tectonic evolution in different stages. The Rushan–Pshart suture zone represents an ancient ocean between Central and Southern Pamir. This paper reports the petrography, geochronology, and geochemistry of Cretaceous granites and diabase dikes that intrude into the Pshart complex. The granites were emplaced between 124 and 118 Ma, based on their zircon U-Pb ages. These granites are characterized by high-K calc-alkaline, low magnesian, and high SiO2, A/CNK, and K2O+Na2O values. They also display strong depletion of Ba, Sr, Eu, and Ti and comparatively weak negative Nb anomalies in spidergrams. Thus, we proposed in this study that these are highly fractionated, strongly peraluminous S-type granites. They were generated by the partial melting of the metasedimentary rocks in the plagioclase stability field and underwent subsequent fractional crystallization during their ascent. The diabase dikes contain low SiO2, and high MgO levels and negative Nb and Ta anomalies, which were interpreted to form in an extensional environment. Late Jurassic–Early Cretaceous closure of the Rushan–Pshart Ocean and subsequent foundering of its oceanic lithosphere caused local extension and upwelling of the asthenospheric mantle. The underplating of mafic magma provided a heat source to melt the metasedimentary-derived granitic that formed in the initial post-collisional environment. The subsequent local extension caused the emplacement of diabase dikes. Based on our new data and combined with data from previous studies, we concluded that the Rushan–Pshart suture zone is the remnant of the Meso-Tethys Ocean and may represent the western continuation of the Bangong–Nujiang suture of the Tibetan Plateau.

Mafic to felsic dyke swarms in coastal Guangdong, China: geochemical and geochronological constraints on the latest Mesozoic geodynamics on the southern margin of the South China Block

AbstractThe coastal region of Guangdong Province, China, is characterized by well-developed dyke swarms. The dykes with widths of decimetres to metres and lengths of tens of metres occur along straight and planar fractures cutting granites or volcanic rocks with Jurassic to Early Cretaceous ages. They show steep attitudes with strikes varying from NNW to NNE and from NE to SE, consistent with a stress regime transition from E–W to N–S extension. Major-element analysis on representative dyke samples reveals a composition range from basaltic to andesitic with a few dacitic outliers. Trace elements of most samples show notable Nb–Ta negative anomalies and Pb positive anomalies on the primitive mantle normalized spidergrams, characteristic of arc-related rocks. Among these, several dacite samples show notable fractionated medium rare earth elements and heavy rare earth elements and high Sr/Y (47–74) and La/Yb (15–21) ratios indicative of adakitic affinity. A few samples lack anomalies in Nb and Ta and have primitive trace-element ratios (e.g. Nb/La and Sr/Nd) or initial ϵNd values resembling ocean island basalt-like rocks. Rare earth element distribution patterns of all samples are right inclined and generally absent of Eu anomalies, which together with other trace-element indexes indicate an origin from heterogeneous mantle sources with depths below the stability field of plagioclase. Radiogenic isotopes, especially those of Nd and Sr (or Nd–Pb), define an array extended from the depleted to enriched mantle (EMII) provenance. LA-ICP-MS dating of zircon obtains ages between 110 and 70 Ma indicating emplacement of these dykes in latest Early to Late Cretaceous time, a time just before initiation of the South China Sea basin. In the context of regional geodynamics, it is proposed that these dykes were formed in a tectonic setting transiting from a Pacific-related back-arc to a passive continental margin pertaining to the development of the South China Sea basin.

Geochronology and geochemistry of the Ediacaran orthogneisses from the north Shahrekord (Sadegh‐Abad), Sanandaj‐Sirjan Zone: Insights into magmatic evolution of the Iranian basement

The metamorphosed Sadegh‐Abad granite from the north Shahrekord Metamorphic Complex (NSMC) of Sanandaj‐Sirjan Zone in west Iran shows evidence of ductile deformation. The augen to banded mylonitic granite (orthogneiss) involves sharp contacts with other rock units, including high‐grade metamorphic rocks, such as eclogite, amphibolite, and garnet amphibolite as well as schists. U–Pb zircon dating reveals that the crystallization of the granitic magma occurred at 568 ± 14 Ma (Late Neoproterozoic, Ediacaran). The Sadegh‐Abad mylonitic granite comprises high SiO2 (67–78 wt.%) and Na2O (3.23–7.79 wt.%) contents and low CaO (0.73–1.61 wt.%) and variable K2O concentrations (0.08–4.6 wt.%). The chondrite‐normalized REE diagram shows enrichment in LREE relative to HREE (Ce/Yb)N = 1.90 to 6.12), a negative Eu anomaly (Eu/Eu* = 0.10–0.66), and nearly flat HREE patterns (Tb/Yb)N = 0.94–1.44). These are interpreted to have originated from a continental crustal source. This is further supported by the high concentration of the alkali elements, U, Th, Rb, and the high 87Sr/86Sr(i) isotopic ratios (0.7086 to 0.7163). The lower HREEs abundances, negative Eu anomalies, and the low contents of mafic minerals may infer that partial melting of continental crust has taken place in low‐pressure conditions within the plagioclase stability field, possibly of pre‐existing calc‐alkaline bodies in the lower continental crust. The charge of heating probably occurred via the injection of mafic magmas into the tonalitic and granodioritic continental crust.

Differential Fluid Activity in a Single Exhumed Continental Subduction Unit from Local P-T-M(H2O) Records of Zoned Amphiboles (North Muya, Eastern Siberia)

The behavior of the continental lithosphere in the Alpine-type subduction zones, which primarily depends on its thickness, thermal regime of subduction and availability of fluids/melts, remains an important issue for both metamorphic petrology and geochemistry as well as for resolving the thermomechanical properties of subduction paleo-interfaces. Rehydrated (amphibole- and zoisite-bearing) eclogites from the Neoproterozoic North Muya high-pressure complex (northern Central Asian Orogenic belt, eastern Siberia) were studied in order to assess their peak burial depths, degree of prograde dehydration, and further retrograde hydration extent within a subducted and exhumed continental unit. Three medium-grained eclogites from different localities of HP complex show similarly dry peak assemblages of pyrope-almandine-grossular garnet (XGrs up to 0.30, XPrp up to 0.25) + Na-rich omphacite (up to 44 mol % of jadeite) + rutile + quartz, which are variably replaced by secondary plagioclase + clinopyroxene ± amphibole symplectitic aggregate. The eclogites were subjected to burial at similar peak depths (up to ~17–21 kbar) but different peak temperatures (within ~600–730 °C) with or without notable heating and re-equilibration due to crustal thickening. Variable degrees of exhumation-induced pervasive rehydration led to growth of individual zoned porphyroblastic barroisite-hornblende amphibole ([B]Na = 0.03–0.45) ± zoisite over the primary eclogitic assemblage or after notable thermally-driven development of symplectitic aggregate after omphacite. Amphibole compositions together with the zoisite presence/absence in different samples reflect continuous rehydration by addition of ~0.5–1.5 wt.% at different exhumation conditions, from nearly peak eclogitic P–T (~17–21 kbar) to granulite- and amphibolite-facies depths within the plagioclase stability field (&lt;14 kbar). This diversity most likely required irregular distribution of internally sourced, low-volume, hydrous metamorphic fluid (i.e., from host felsic rocks or metasediments) acting at different depths of the subduction interface. From the performed PTX calculations, I suggest that nearly isochemical (i.e., without any significant modification of the bulk-rock composition other than incorporation of additional H2O), retrograde hydration by only at lower- to middle-crust conditions did not significantly influence the density and the rheology of the subducted continental slices due to both (1) a limited abundance of dense metabasic rocks, which are commonly more fluid-rich (e.g., due to chlorite or amphibole alteration), and (2) the initially dry nature of mafic and felsic continental rocks. The limited dehydration and rehydration scales exemplified by the North Muya eclogites and therefore low availability of hydrous metamorphic fluids may have accounted for the high buoyancy of the eclogitic crust and explained the absence of contemporaneous suprasubduction magmatism in the regional context at ca. ~630 Ma.

Polybaric fractional crystallisation of arc magmas: an experimental study simulating trans-crustal magmatic systems

Crystallisation-driven differentiation is one fundamental mechanism proposed to control the compositional evolution of magmas. In this experimental study, we simulated polybaric fractional crystallisation of mantle-derived arc magmas. Various pressure–temperature trajectories were explored to cover a range of potential magma ascent paths and to investigate the role of decompression on phase equilibria and liquid lines of descent (LLD). Fractional crystallisation was approached in a step-wise manner by repetitively synthesising new starting materials chemically corresponding to liquids formed in previous runs. Experiments were performed at temperatures ranging from 1140 to 870 °C with 30 °C steps, and pressure was varied between 0.8 and 0.2 GPa with 0.2 GPa steps. For most fractionation paths, oxygen fugacity (fO2) was buffered close to the Ni-NiO equilibrium (NNO). An additional fractionation series was conducted at fO2 corresponding to the Re-ReO2 buffer (RRO ≈ NNO+2). High-pressure experiments (0.4–0.8 GPa) were run in piston cylinder apparatus while 0.2 GPa runs were conducted in externally heated pressure vessels. Resulting liquid lines of descent follow calc-alkaline differentiation trends where the onset of pronounced silica enrichment coincides with the saturation of amphibole and/or Fe–Ti–oxide. Both pressure and fO2 exert crucial control on the stability fields of olivine, pyroxene, amphibole, plagioclase, and Fe–Ti–oxide phases and on the differentiation behaviour of arc magmas. Key observations are a shift of the olivine–clinopyroxene cotectic towards more clinopyroxene-rich liquid composition, an expansion of the plagioclase stability field and a decrease of amphibole stability with decreasing pressure. Decompression-dominated ascent trajectories result in liquid lines of descent approaching the metaluminous compositional range observed for typical arc volcanic rocks, while differentiation trends obtained for cooling-dominated trajectories evolve to peraluminous compositions, similar to isobaric liquid lines of descent at elevated pressures. Experiments buffered at RRO provide a closer match with natural calc-alkaline differentiation trends compared to fO2 conditions close to NNO. We conclude that decompression-dominated fractionation at oxidising conditions represents one possible scenario for arc magma differentiation.

Sodic-Silicic Magmatism in Tadpatri Formation- A Study in Quartz Keratophyre and Albitite

Abstract The mafic sills and unmetamorphosed shales and siltstones of Tadpatri Formation in Cuddapah Supergroup are associated with thin flows of quartz keratophyre and an occasional albitite dyke. The textural characteristics of these quartz keratophyre essentially establish them as igneous extrusive rocks. A detailed study of texture and chemistry, coupled with comparison of available records in geological literature, suggests that these rocks were derived through very small degree of partial melting of metasomatized mantle at pressures more than plagioclase stability field and less than jadeite stability field. High alkali content and associated volatiles led to significant decrease in viscosity and emplacement of these lavas in an extensional tectonic set-up.

Rhyolites in continental mafic Large Igneous Provinces: Petrology, geochemistry and petrogenesis

We present a detailed review of the petrological and geochemical aspects of rhyolite and associated silicic volcanic rocks (up to 20 ​vol% of all rocks) reported to date from twelve well known Phanerozoic continental mafic Large Igneous Provinces (LIPs). These typically spread over ≤104 ​km2 (rarely 105 ​km2 for Paraná-Etendeka) area and comprise ≤104 ​km3 of extrusive silicic rocks, erupted either during or after the main basaltic eruption within <5 Myr, with some eruption(s) continuing for ≤30 Myr. These rhyolites and associated silicic volcanic rocks (60−81 ​wt.% of SiO2) are mostly metaluminous to peraluminous and are formed via (i) fractional crystallization of parental mafic magma with negligible crustal contamination, and (ii) melting of continental crust or assimilation and fractional crystallization (AFC) of mafic magma with significant crustal contribution. Rhyolites formed by extensive fractional crystallization are characterized by the presence of clinopyroxene phenocrysts, exhibit steep negative slopes in bivariate major oxides plots and weak to no Nb-Ta anomaly; these typically have temperature >900 ​°C. Rhyolites formed by significant crustal contribution are characterized by strong negative Nb-Ta anomalies, absence of clinopyroxene phenocrysts, and are likely to have a magma temperature <900 ​°C. Geochemical signatures suggest rhyolite melt generation in the plagioclase stability field with a minor fraction originating from lower crustal depths. A large part of the compositional variability in rhyolites, particularly the Sr-Nd-Pb-O isotope ratios, suggests a significant role of continental crust (upper crustal melting or AFC) in the evolution of these silicic rocks in the continental mafic LIPs.

Up to 1% Pb isotope disequilibrium between minerals hosted in dacites from the Guagua Pichincha volcano, Ecuador: Implication for tracing the source and crustal history of continental arc magmas

Continental arc lavas display geochemical signatures that reflect both mantle metasomatism by slab fluids or melts and extensive differentiation of magmas within crustal reservoirs. The relative effect of source and crustal processes are difficult to disentangle based on whole-rock compositions. This issue is critical in Ecuador where volcanism occurs through a thick continental crust (>50 km). This study reconstructs the history of melts feeding the Guagua Pichincha volcano, Western Cordillera, by analysing the Pb isotope composition and major-trace element content of individual minerals (33 amphiboles, 4 orthopyroxenes and 18 plagioclases) hosted in two dacite samples. It uses a low-blank wet-chemistry method for precise analysis of Pb amounts as low as 150 pg.Early crystallized, high-Al amphiboles with Al2O3 ≥ 9.8 wt% and Eu/Eu* > 0.7 have the lowest and most heterogeneous 206Pb/204Pb (18.816–18.999), whereas plagioclases have the highest and most homogeneous 206Pb/204Pb (19.003–19.023). Low-Al amphiboles and orthopyroxenes display intermediate compositions and variability (18.934–19.007). The 206Pb/204Pb ratio correlates negatively with Eu/Eu* in amphiboles and orthopyroxenes, which indicates that the Guagua Pichincha magmas assimilate radiogenic Pb within the stability field of plagioclase (i.e. in the upper crust). The radiogenic ankaramites of the Guaranda unit, an accreted ocean terrain making the basement of the Western Cordillera, are the most suitable contaminant. If this is correct, the 206Pb/204Pb increase from the two most primitive amphiboles to their respective host rocks requires ca. 20% crustal assimilation, which is higher than previous estimates in the Northern Volcanic Zone but similar to those inferred for Central Andean mafic lavas. The two most primitive amphiboles with no significant Eu anomaly record the composition of melts before plagioclase crystallization. These deep melts have contrasted 206Pb/204Pb ratios (18.816–18.879) and contents of fluid mobile elements (Li, Cu, Rb, Pb) that probably reflect the input of different slab components to the mantle wedge. Melts in equilibrium with the two most primitive amphiboles of the Guagua Pichincha volcano are enriched in incompatible elements, but depleted in fluid mobile elements compared to the olivine-hosted melt inclusions of the older Rucu Pichincha volcano. This supports previous inferences based on whole-rock data that the mantle source of the Pichincha Volcanic Complex has changed through time.

Modeling the Crystallization and Emplacement Conditions of a Basaltic Trachyandesitic Sill at Mt. Etna Volcano

This study documents the compositional variations of phenocrysts from a basaltic trachyandesitic sill emplaced in the Valle del Bove at Mt. Etna volcano (Sicily, Italy). The physicochemical conditions driving the crystallization and emplacement of the sill magma have been reconstructed by barometers, oxygen barometers, thermometers and hygrometers based on clinopyroxene, feldspar (plagioclase + K-feldspar) and titanomagnetite. Clinopyroxene is the liquidus phase, recording decompression and cooling paths decreasing from 200 to 0.1 MPa and from 1050 to 940 °C, respectively. Plagioclase and K-feldspar cosaturate the melt in a lower temperature interval of ~1000–870 °C. Cation exchanges in clinopyroxene (Mg-Fe) and feldspar (Ca-Na) indicate that magma ascent is accompanied by progressive H2O exsolution (up to ~2.2 wt. %) under more oxidizing conditions (up to ΔNNO + 0.5). Geospeedometric constraints provided by Ti–Al–Mg cation redistributions in titanomagnetite indicate that the travel time (up to 23 h) and ascent velocity of magma (up to 0.78 m/s) are consistent with those inferred for other eruptions at Mt. Etna. These kinetic effects are ascribed to a degassing-induced undercooling path caused principally by H2O loss at shallow crustal conditions. Rare earth element (REE) modeling based on the lattice strain theory supports the hypothesis that the sill magma formed from primitive basaltic compositions after clinopyroxene (≤41%) and plagioclase (≤12%) fractionation. Early formation of clinopyroxene at depth is the main controlling factor for the REE signature, whereas subsequent degassing at low pressure conditions enlarges the stability field of plagioclase causing trace element enrichments during eruption towards the surface.

P – T conditions of symplectite formation in the eclogites from the Western Gneiss Region (Norway)

Abstract Symplectite, defined as plagioclase + Ca-pyroxene (±amphibole) intergrowths after omphacite, and kelyphite, defined as amphibole + plagioclase coronas around garnet, are common features of retrogressed eclogites. These textures are related to exhumation under (ultra) high pressure towards the surface, but the estimation of the pressure–temperature ( P – T ) of symplectite formation is difficult because of the narrowness of pyroxene and plagioclase lamellas, and the compositional variability of the phases. Retrogressed eclogites from Norwegian localities with different eclogite peak conditions have been chosen to investigate the formation of symplectite and associated kelyphite. Thermobarometry calculations show that symplectite crystallizes as soon as the rocks enter the stability field of plagioclase and continues crystallizing until they have reached amphibolite facies. Symplectite yields a pressure range from 18 to 10 kbar, and a temperature range from 700 to 550°C. Amphibole found in the symplectite assemblage crystallizes later, at lower pressures and temperatures (10–4 kbar, 680–420°C). Kelyphite is always associated with well-developed symplectite, when the former omphacite is totally transformed into symplectite. These features likely testify to the influence of an external fluid during retrogression. Samples with limited symplectite and no kelyphite are likely retrogressed with an internal fluid.

Dual origins for pantellerites, and other puzzles, at Mount Takahe volcano, Marie Byrd Land, West Antarctica

Mt. Takahe is a large, late Quaternary trachyte shield volcano that rises through 2000+m of the West Antarctic ice sheet. It is composed mostly of ne-trachyte, hy-ol-trachyte, and qz-trachyte flows, with subordinate basanite, intermediate rocks, and pantellerites.All rock types can be adequately modeled by fractional crystallization of basanite - the only basaltic rock exposed here. The ne-trachytes can be explained by a single stage of low-pressure fractionation near the base of the upper crust. Models of oversaturated rocks require a period of evolution at a depth of ~35km, below the stability field of plagioclase, where fractionation of kaersutite and associated high pressure minerals will yield silica oversaturated residual magmas. This is then followed by a period of fractionation at a depth of ~3km, where peralkalinity and Fe-enrichment are acquired. Pantellerite compositions span virtually the entire spectrum of peralkalinity, Fe-enrichment, LILE–enrichment, and SiO2 values, and seem to represent a range of residence times in upper crustal vs., upper mantle magma chambers.Mt. Takahe is unusual among Marie Byrd Land volcanoes for its geochemical anomalies. These include the lowest 143Nd/144Nd ratios in West Antarctica, and unusually high but unpredictable Ba values. These anomalies are believed to originate in a pre-85Ma subduction mélange at the base of the lithosphere, which seems to be the source of Mt. Takahe basaltic rocks.

Large-scale removal of lithosphere underneath the North China Craton in the Early Cretaceous: Geochemical constraints from volcanic lavas in the Bohai Bay Basin

Cratons are generally considered as the most stable tectonic units on the Earth. Rare magmatism, seismic activity, and intracrustal ductile deformation occur in them. However, several cratons experienced entirely different fates, including the North China Craton (NCC), and were subsequently destroyed. Geodynamic mechanisms and timing of the cratonic destruction are strongly debated. In this paper, we investigate a suite of Mesozoic intermediate to felsic volcanic rocks which are collected from boreholes in the Liaohe Depression of the Bohai Bay Basin the eastern NCC. These volcanic rocks have Precambrian basement-like Sr–Nd isotopic characteristics, consistent with derivation from the lower continental crust underneath the NCC. The Late Jurassic (~165Ma) intermediate volcanic rocks don't exhibit markedly negative Eu anomalies, which require a source beyond the plagioclase stability field. And the low heavy rare earth elements (HREEs) contents of these samples indicate that their source has garnet as residue. The Early Cretaceous (~122Ma) felsic volcanic rocks are depleted in HREEs but with remarkable Eu anomalies, suggesting that their source have both garnet and plagioclase. The crust thicknesses, estimated from the geochemistry of the intermediate and felsic rocks, are ≥50km at ~165Ma and 30–50km at ~122Ma, respectively. The crustal thinning is attributed to lithospheric delamination beneath the NCC. Our results combined with previous studies imply that the large-scale lithospheric removal occurred in the Early Cretaceous, between 140 and 120Ma.

Experimental calibration of Forsterite–Anorthite–Ca-Tschermak–Enstatite (FACE) geobarometer for mantle peridotites

The crystallization of plagioclase-bearing assemblages in mantle rocks is witness of mantle exhumation at shallow depth. Previous experimental works on peridotites have found systematic compositional variations in coexisting minerals at decreasing pressure within the plagioclase stability field. In this experimental study we present new constraints on the stability of plagioclase as a function of different Na2O/CaO bulk ratios, and we present a new geobarometer for mantle rocks. Experiments have been performed in a single-stage piston cylinder at 5–10 kbar, 1050–1150 °C at nominally anhydrous conditions using seeded gels of peridotite compositions (Na2O/CaO = 0.08–0.13; X Cr = Cr/(Cr + Al) = 0.07–0.10) as starting materials. As expected, the increase of the bulk Na2O/CaO ratio extends the plagioclase stability to higher pressure; in the studied high-Na fertile lherzolite (HNa-FLZ), the plagioclase-spinel transition occurs at 1100 °C between 9 and 10 kbar; in a fertile lherzolite (FLZ) with Na2O/CaO = 0.08, it occurs between 8 and 9 kbar at 1100 °C. This study provides, together with previous experimental results, a consistent database, covering a wide range of P–T conditions (3–9 kbar, 1000–1150 °C) and variable bulk compositions to be used to define and calibrate a geobarometer for plagioclase-bearing mantle rocks. The pressure sensitive equilibrium: $$\mathop {{\text{M}}{{\text{g}}_{\text{2}}}{\text{Si}}{{\text{O}}_{\text{4}}}^{{\text{Ol}}}}\limits_{{\text{Forsterite}}} +\mathop {{\text{CaA}}{{\text{l}}_{\text{2}}}{\text{S}}{{\text{i}}_{\text{2}}}{{\text{O}}_{\text{8}}}^{{\text{Pl}}}}\limits_{{\text{Anorthite}}~} =\mathop {{\text{CaA}}{{\text{l}}_{\text{2}}}{\text{Si}}{{\text{O}}_{\text{6}}}^{{\text{Cpx}}}}\limits_{{\text{Ca-Tschermak}}} +{\text{ }}\mathop {{\text{M}}{{\text{g}}_{\text{2}}}{\text{S}}{{\text{i}}_{\text{2}}}{{\text{O}}_{\text{6}}}^{{\text{Opx}}}}\limits_{{\text{Enstatite}}} ,$$ has been empirically calibrated by least squares regression analysis of experimental data combined with Monte Carlo simulation. The result of the fit gives the following equation: $$P=7.2( \pm 2.9)+0.0078( \pm 0.0021)T{\text{ }}+0.0022( \pm 0.0001)T{\text{ }}\ln K,$$ $${R^2}=0.93,$$ where P is expressed in kbar and T in kelvin. K is the equilibrium constant K = a CaTs × a en/a an × a fo, where a CaTs, a en, a an and a fo are the activities of Ca-Tschermak in clinopyroxene, enstatite in orthopyroxene, anorthite in plagioclase and forsterite in olivine. The proposed geobarometer for plagioclase peridotites, coupled to detailed microstructural and mineral chemistry investigations, represents a valuable tool to track the exhumation of the lithospheric mantle at extensional environments.

Geochemistry of the Palaeoproterozoic volcanic and associated potassic granitic rocks of the Ngualla area of the Ubendian Belt, SW Tanzania

The Ngualla area of the Lupa terrane in the Ubendian Belt of SW Tanzania is underlain by well-preserved greenschist facies calc-alkaline volcanic rocks which have been intruded by low-Sr, peraluminous K-rich granites. The Ngualla volcanic rocks are subdivided into four groups: the basaltic andesites-trachyandesites, Group I porphyritic dacites-trachytes, Group II porphyritic dacites-trachytes, and rhyolites. The basaltic andesites-trachyandesites were emplaced at ∼1943 Ma and their most primitive members are characterized by MgO contents of up to 7.43 wt % at a SiO2 content of 54.0 wt %, Ni contents of up to 170 ppm and Mg# of up to 62. They show fractionated REE patterns (La/YbCN = 11.5–36.5), sub-chondritic Zr/Hf (37.8–43.9) and Nb/Ta (7.14–20.0) ratios, εNd (1943 Ma) values of −3.33 to −6.24 and TDM ages of 2521–2883 Ma. These geochemical features are consistent with derivation of the basaltic andesites-trachyandesites by at least 0.1% partial melting of garnet peridotite mantle, followed by 50–80% fractional crystallization involving the removal of 61% clinopyroxene, 20% hornblende, 4% plagioclase and 15% orthopyroxene. The magmas were contaminated by Neoarchaean crustal materials incorporated in the Ubendian Belt during their ascent to the surface. The evolution of these magmas led to the formation of Group I porphyritic dacites-trachytes and rhyolites in a Palaeoproterozoic continental arc setting.The Group II porphyritic dacites-trachytes (1871 ± 5 Ma) were coevally emplaced with the low-Sr, peraluminous K-granites (1878 ± 15 Ma) forming a felsic plutonic-volcanic suite. Despite some differences, these rocks share geochemical features including negative Eu anomalies (mean Eu/Eu* = 0.19 for the dacites and 0.5 for the granites) and εNd (1871 Ma) values of −5.72 for the dacites and εNd (1878 Ma) values of −6.00 to −11.2 for the granites. These geochemical features are consistent with the generation of the ∼1.87 Ga Ngualla felsic plutonic-volcanic suite by partial melting of heterogeneous Neoarchaean crust at low pressure conditions in the stability field of plagioclase in an intracontinental setting along a sutured margin.The 1.89–1.87 Ga felsic plutonic-volcanic suite also occurs in the Ndembera area of the Usagaran belt to the SW. Our results corroborate previous evidence that the Palaeoproterozoic regional K-granitic magmatism that affected parts of the Ubendian and Usagaran belts was caused by a regional thermal anomaly that was induced by large scale magmatic underplating that precipitated widespread crustal anatexis.

Geochronology and petrogenesis of shoshonitic igneous rocks, Luzong volcanic basin, middle and lower Yangtze River reaches, China

The Luzong volcanic basin has a unique tectonic background, being located at the northeastern margin of the Yangtze craton and the foreland of the Dabie–Sulu orogen, near the intersection of the renowned Tanlu and Yangtze River faults. The volcanic and subvolcanic rocks in the basin are characterized by high potassium contents and are rich in alkali elements, and belong to a typical shoshonite series. Geochemically, these rocks are depleted in high field strength elements such as Nb and Ta, and enriched in highly incompatible elements such as Rb, Th, U, K, and light rare earth elements. Nd and Sr isotope compositions, combined with elemental geochemical characteristics, indicate that the parental magmas were formed mainly by partial melting of enriched subcontinental lithospheric mantle. The magmas underwent minor or negligible contamination by upper crustal rocks or fractional crystallization at low pressures (i.e., <1.5GPa). Nevertheless, variation trends for major and trace elements indicate that these shoshonitic mantle-derived magmas experienced fractional crystallization at high pressures (i.e., above the stability field of plagioclase, >1.5GPa), dominated by the removal of clinopyroxene and Fe–Ti oxides. The volcanic rocks of the Luzong Basin can be divided into four formations, which, from early to late, are the Longmenyuan, Zhuanqiao, Shuangmiao, and Fushan Formations, each separated by clear gaps in eruptive activity. In this study, we obtained zircon U–Pb SHRIMP ages from two subvolcanic samples from the Longmenyuan and Zhuanqiao Formations, which yield emplacement ages of 132±1Ma and 131±1Ma. These data, combined with ages reported by other workers, show that the duration of volcanism in the Luzong Basin (and the whole middle and lower Yangtze River reaches) was very short (⩽8Ma). We consider that in the Early Cretaceous the foreland area of the Dabie orogenic belt was under extension, influenced by the relative motion of the Pacific and Eurasian plates, and by sinistral displacement along the Tanlu Fault. At this time, rapid mechanical delamination of thickened lithosphere caused lithospheric thinning and resultant asthenospheric upwelling in the middle and lower Yangtze River region and the Dabie orogenic belt. These deep geological and tectonic events in turn produced the shoshonite volcanic rocks in the Luzong Basin.

Increased mantle heat flow with on-going rifting of the West Antarctic rift system inferred from characterisation of plagioclase peridotite in the shallow Antarctic mantle

The lithospheric, and shallow asthenospheric, mantle in Southern Victoria Land are known to record anomalously high heat flow but the cause remains imperfectly understood. To address this issue plagioclase peridotite xenoliths have been collected from Cenozoic alkalic igneous rocks at three localities along a 150km transect across the western shoulder of the West Antarctic rift system in Southern Victoria Land, Antarctica. There is a geochemical, thermal and chronological progression across this section of the rift shoulder from relatively hot, young and thick lithosphere in the west to cooler, older and thinner lithosphere in the east. Overprinting this progression are relatively more recent mantle refertilising events. Melt depletion and refertilisation was relatively limited in the lithospheric mantle to the west but has been more extensive in the east. Thermometry obtained from orthopyroxene in these plagioclase peridotites indicates that those samples most recently affected by refertilising melts have attained the highest temperatures, above those predicted from idealised dynamic rift or Northern Victoria Land geotherms and higher than those prevailing in the equivalent East Antarctic mantle. Anomalously high heat flow can thus be attributed to entrapment of syn-rift melts in the lithosphere, probably since regional magmatism commenced at least 24Myr ago. The chemistry and mineralogy of shallow plagioclase peridotite mantle can be explained by up to 8% melt extraction and a series of refertilisation events. These include: (a) up to 8% refertilisation by a N-MORB melt; (b) metasomatism involving up to 1% addition of a subduction-related component; and (c) addition of ~1.5% average calcio-carbonatite. A high MgO group of clinopyroxenes can be modelled by the addition of up to 1% alkalic melt. Melt extraction and refertilisation mainly occurred in the spinel stability field prior to decompression and uplift. In this region mantle plagioclase originates by a combination of subsolidus recrystallisation during decompression within the plagioclase stability field and refertilisation by basaltic melt.