High-pressure Low-temperature Metamorphism Research Articles

Lithium pegmatite of anatectic origin – A case study from the Austroalpine Unit Pegmatite Province (Eastern European Alps): Geological data and geochemical modeling

There is an ongoing debate as to whether rare element pegmatite is always related to fractional crystallization of huge fertile granite bodies or whether it can also form directly from limited portions of enriched anatectic melt. In this contribution, we present a case study from the Eastern European Alps, showing the continuous evolution from melt generation in staurolite bearing micaschist to spodumene (LiAlSi2O6) bearing pegmatite. The investigated Austroalpine Unit Pegmatite Province formed during Permian lithospheric extension and all levels of the Permian crust are now accessible in a Cretaceous nappe stack. This nappe stack is mapped in detail, subdivided lithostratigraphically and the internal tectonic structure is well understood. It is clear that the described pegmatites are neither spatially nor genetically related to large fertile granite bodies. A review of geochronological data proofs that emplacement of pegmatite and leucogranite is broadly contemporaneous with high temperature-low pressure metamorphism of the country rocks. Field observations give clear evidence of a genetic link between simple pegmatite, leucogranite, evolved pegmatite and albite-spodumene pegmatite, on the one hand, and the subsolidus or suprasolidus metasediment hosting them on the other hand. These relations are underpinned by geochemical major and trace element investigations on all types of rock and the minerals therein. As source rock an Al-rich metapelite enriched in Li (70–270 ppm) with respect to the average upper continental crust is identified. The main Li-carrier is staurolite with up to 3000 ppm Li. The pegmatite and leucogranite originate in anatectic melts that formed between 0.6 and 0.8 GPa and 650–750 °C, corresponding 18–26 km depth. During melt formation staurolite was consumed by sillimanite forming reactions. Subsequently, the melts were enriched in Li by fractional crystallization of quartz and feldspar during their ascent to higher crustal levels. While simple pegmatite and inhomogeneous leucogranite formed in lower and intermediate levels, evolved and albite-spodumene pegmatite crystallised at high levels at conditions between 0.3 and 0.4 GPa and 500–570 °C, corresponding to about 12 km depth. Based on these data we develop a geochemical model for showing that Li can be transferred from a metasediment into an anatectic melt if staurolite is stable or metastable at the onset of melting. Such a melt can contain between 200 and 1000 ppm Li and the melt can be further enriched with up to 5000 to 10000 ppm Li through fractional crystallization of quartz and feldspar, allowing crystallization of spodumene. Estimated fractionation degrees vary between 81 and 99 %, depending on the protolith composition, the melting scenario and the partitioning coefficients. Li-Al-rich metasediments are therefore a rich source of certain rare elements when they first melt. However, the spectrum of elements contained in the melt depends on a wide variety of conditions. This anatectic model provides an alternative explanation for the formation of Li-rich pegmatite if no fertile parent granite can be identified.

Raman thermometry and (U-Th)/He thermochronometry reveal Neogene transpressional exhumation in the Nacimiento block of central California, USA

Abstract We present a novel approach for mapping vertical uplifts in exhumed metasedimentary rocks by coupling Raman spectroscopy of carbonaceous material with (U-Th)/He thermochronometry on apatite and zircon. We apply this approach to carbonaceous metasedimentary rocks of the Franciscan subduction complex, exposed in the Nacimiento block of central California, USA, an area that records high-pressure–low-temperature metamorphism prior to entrainment within the present-day transform plate boundary. We reveal the extent and magnitude of previously unrecognized exhumation gradients, which, combined with regional structural observations, can be used to quantify vertical crustal motion associated with localized transpression. We propose that the Nacimiento block was affected by a kilometer-scale, post-subduction thermal anomaly linked to a localized transpressive regime since ca. 25 Ma, with an uplift rate of ∼0.3 mm/yr.

Prolonged high thermal gradient metamorphism in the Curnamona Province, south-central Australia, during the latter stages of Nuna assembly

Granulite-facies metasedimentary rocks from the Broken Hill Block of the Curnamona Province, south-central Australia, record a prolonged period (c. 1630–1550 Ma) of high thermal gradient metamorphism and anatexis. We present monazite and zircon U–Pb geochronological and REE geochemical data from garnet-bearing paragneisses of the Willyama Supergroup, coupled with garnet trace element mapping and mineral equilibria modelling, to develop a petrochronological model for the Palaeo–Mesoproterozoic metamorphic evolution of this region. Our results indicate that the Curnamona Province underwent an early phase of high-temperature–low-pressure metamorphism between c. 1630–1610 Ma. Peak metamorphism involving wholesale anatexis subsequently occurred under an apparent thermal gradient of 150–170 °C/kbar at c. 1600–1580 Ma during regional compression. Monazite U–Pb dates reveal that suprasolidus conditions were maintained for at least 30 Myr, with final melt crystallisation potentially occurring as late as c. 1550 Ma, indicating that the system remained hot for an extended duration. We suggest that high thermal gradient metamorphism in the Willyama Supergroup was driven by elevated levels of radiogenic heat production hosted within the Willyama succession. Temperatures may have been initially boosted by low-to-moderate degrees of crustal extension, although there is a dearth of coeval magmatism and structural evidence to support this claim. Peak metamorphism coincides with the timing of compressional orogenesis and related metamorphism in other Proterozoic terranes that are inferred to have occupied the eastern margin of proto-Australia. Many of these terranes also exhibit prolonged metamorphic histories and elevated levels of radiogenic heat production, suggesting that radiogenic heating may have had a major influence in pre-disposing these terranes to be the locus of metamorphism and deformation during the final assembly of Nuna.

P-T paths and U-Pb ages of pelitic and semi-pelitic granulites in the Yunkai massif and implication for the tectonic evolution of the Wuyi-Yunkai orogen, South China

Understanding the tectono-metamorphic evolution of the Wuyi-Yunkai orogen is crucial to constraining the Early Paleozoic geodynamic history of the South China Block. Here, we present petrological and geochronological characteristics of the pelitic and semi-pelitic granulites from Yunkai massif that occur as lenses or enclaves within orthogneisses and charnockites. These granulites have garnet-cordierite-spinel-sillimanite-bearing and garnet-orthopyroxene-bearing assemblages, respectively, showing typical low-pressure–high-temperature (LP-HT) granulite-facies mineral assemblages. Based on petrological characteristics, P-T calculations and phase equilibrium modelling in the NCKFMASHTO system, three-stage metamorphic evolution can be distinguished: 1) prograde stage (M1) characterized by biotite-plagioclase- or sillimanite-bearing assemblages present as inclusions in garnet or cordierite porphyroblasts, with P-T conditions of 7.0–8.0 kbar/730–780 °C; 2) peak LP-HT stage (M2) featured by garnet-orthopyroxene- or cordierite-bearing assemblages, with P-T conditions of 4.7–5.1 kbar/825–875 °C, suggesting high geothermal gradients (∼50–56 °C/km); and 3) post-peak stage (M3) represented by garnet+quartz corona or spinel-cordierite-bearing intergrowth, with P-T conditions of 4.0–4.3 kbar/650–750 °C. P-T estimates and pseudosection modelling define two consistent clockwise P-T paths involving a pre-peak prominent heating with decompression followed by a post-peak nearly isobaric cooling process. LA-ICP-MS zircon U-Pb dating results reveal that the granulite-facies metamorphism occurred at ∼445 Ma. Combined with previous studies, we proposed that the LP-HT metamorphism in the Yunkai massif might be attributed to the ridge subduction in the Early Paleozoic, which caused asthenosphere upwelling and supplied high heat flux to the lower crust when the slab window opened.

Proterozoic High-Temperature–Low-Pressure Metamorphism in the Mahakoshal Belt, Central Indian Tectonic Zone (India): Structure, Metamorphism, U-Th-Pb Monazite Geochronology, and Tectonic Implications

To understand the protracted accretionary evolution along the Central Indian Tectonic Zone (CITZ), we investigated garnet-staurolite schists and associated lithologies from the central Mahakoshal Belt (MB). Mesoscale structures, porphyroblast growth, garnet zoning, and pseudosection modeling were coupled with U-Th-Pb monazite dating to reconstruct a clockwise pressure-temperature-time (P-T-t) path for the garnet- and staurolite-bearing schists. The prograde path is characterized by near isobaric heating conditions, which initiates at pressure-temperature (P-T) conditions of 4.5–5.0 kbar and 550°–560°C and attains peak metamorphism at 5.5–6.0 kbar and 610°–620°C. The peak metamorphism was contemporaneous with the emplacement of southern-margin granitoids at ~1.70 Ga, resulting in the perturbation of geotherms in the collision setting. The retrograde arm (MR) of the P-T path passes through isobaric cooling at ~5.0 kbar and ~500°C. This late Paleoproterozoic P-T-t path is overprinted by hitherto uncharacterized tectonism that coincides with the Sausar orogeny and provides evidence for the northward extension of 0.95–0.85 Ga orogenic activity within the CITZ. The 1.80–1.55 and 0.95–0.85 Ga tectonothermal events identified in this study support that crustal evolution in the CITZ involved a mosaic of domains that were accreted together in the Neoproterozoic time.

Extrusion of subducted crust explains the emplacement of far-travelled ophiolites

Continental subduction below oceanic plates and associated emplacement of ophiolite sheets remain enigmatic chapters in global plate tectonics. Numerous ophiolite belts on Earth exhibit a far-travelled ophiolite sheet that is separated from its oceanic root by tectonic windows exposing continental crust, which experienced subduction-related high pressure-low temperature metamorphism during obduction. However, the link between continental subduction-exhumation dynamics and far-travelled ophiolite emplacement remains poorly understood. Here we combine data collected from ophiolite belts worldwide with thermo-mechanical simulations of continental subduction dynamics to show the causal link between the extrusion of subducted continental crust and the emplacement of far-travelled ophiolites. Our results reveal that buoyancy-driven extrusion of subducted crust triggers necking and breaking of the overriding oceanic upper plate. The broken-off piece of oceanic lithosphere is then transported on top of the continent along a flat thrust segment and becomes a far-travelled ophiolite sheet separated from its root by the extruded continental crust. Our results indicate that the extrusion of the subducted continental crust and the emplacement of far-travelled ophiolite sheets are inseparable processes.

Distribution and intensity of High-Temperature Low-Pressure metamorphism across the Pyrenean-Cantabrian belt: constraints on the thermal record of the pre-orogenic hyperextension rifting

Whereas a straightforward link between crustal thinning and geothermal gradients during rifting is now well established, the thermal structure of sedimentary basins within hyperextended domains remains poorly documented. For this purpose, we investigate the spatial distribution of rift-related High-Temperature Low-Pressure (HT/LP) metamorphism recorded in the preserved hyperextended rift basins inverted and integrated in the Pyrenean-Cantabrian belt. Based on Vitrinite Reflectance (Ro) data measured in 169 boreholes and more than 200 peak-metamorphic temperatures (Tmax) data obtained by Raman Spectroscopy of Carbonaceous Material (RSCM) added to ∼425 previously publishedTmaxdata, we propose a new map depicting the spatial distribution of the HT/LPmetamorphism of the Pyrenean-Cantabrian belt. We also provide three regional-scale geological cross-sections associated with RoandTmaxdata to constrain the distribution of paleo-isograds at depth. Based on these results, we show that the impact of rift-related metamorphism is restricted to the pre- and syn-rift sequence suggested by the depth profiles of Rovalues measured in different tectonostratigraphic intervals (pre-, syn- and post-rift and syn-convergence sediments). However, a small strip of early orogenic sediments (Santonian in age) appears also affected by high temperatures along the North Pyrenean Frontal Thrust and above the Grand Rieu ridge, which we attribute to the percolation of hot hydrothermal fluids sourced from the dehydration of underthrust basement and/or sedimentary rocks at depth during the early orogenic stage. The map shows that the HT/LPmetamorphism (reaching ∼500 °C) is recorded with similar intensity along the Pyrenean-Cantabrian belt from the west in the Basque-Cantabrian Basin to the east in the Boucheville and Bas-Agly basins, for similar burial and rift-related structural settings. This thermal peak is also recorded underneath the northern border of the Mauléon Basin (calibrated by wells). It suggests that the high temperatures were recorded at the basement-sediment interface underneath the most distal part of the hyperextended domain. At basin-scale, we observe in the Basque-Cantabrian, Mauléon-Arzacq and Tarascon rift segments an asymmetry of the thermal structure revealed by different horizontal thermal gradients, supporting an asymmetry of the former hyperextended rift system. Using our results, we compare the Pyrénées to the Alps that also recorded hyperextension but no HT/LPmetamorphic event and suggest that the high-temperature record within the basins depends on high sedimentation rate promoting a thermal blanketing effect and circulation of hydrothermal fluids.

Evaporitic brines and copper-sulphide ore genesis at Jbel Haïmer (Central Jebilet, Morocco)

The Jbel Haïmer copper-sulphide mineralisation occurs at 20 km north of Marrakesh in the Variscan Jebilet massif, Morocco. Most of the ores (up to 3.9 wt% Cu, ≤38 ppm Ag, and up to 2.9 ppm Au) occur as impregnations of NE-SW fault/fracture zones and related tectonic breccia. Two independent stages of fluid circulation and mineral deposition are distinguished. First, Late Variscan high temperature-low pressure metamorphism synchronous of granite intrusion induced percolation of C–H-O-N hydrothermal fluids throughout faults and shear zones. The drop in pressure from lithostatic down to hydrostatic values at temperatures around 350 ± 50 °C triggered quartz precipitation associated with minor Sn-As-(Co-Ni) deposits and finally brecciation. The second stage, more recent than Triassic, consists in the deposition of quartz + carbonates, followed by Cu-(Pb-Zn) sulphides (±Ag-Au alloys) in fractures crosscutting 240 Ma microdiorite dikes. Mixing of evaporitic brines likely coming from Triassic formations with low salinity aqueous fluids was responsible for Cu-sulphide deposition at temperatures of around 220–280 °C at a depth of 4–5 km. These base metal-rich brines are similar to those from the nearby Roc Blanc Ag-deposit and several other silver and base metal deposits in Morocco, considered as having circulated during the Central Atlantic Ocean opening.

Paleogeography of Late Jurassic large-igneous-province activity in the Paleo-Pacific Ocean: Constraints from the Mikabu greenstones and Chichibu accretionary complex, Kanto Mountains, Central Japan

The Mikabu Unit (Mikabu greenstones) was an oceanic plateau that was in the Mesozoic Paleo-Pacific Ocean. Large-igneous-province (LIP) activity has been proposed as the origin of the oceanic plateau of the Mikabu Unit. This study aims to reveal LIP activity in the Paleo-Pacific Ocean. We present geochemical and geochronological data for the Mikabu Unit and the Middle Jurassic to Early Cretaceous Chichibu accretionary complex (i.e., the Kashiwagi and Kamiyoshida units), both of which underwent the same high-pressure low-temperature metamorphism as the Mikabu Unit, in the Kanto Mountains, central Japan. The Mikabu Unit contains oceanic plateau basalts characterized by incompatible element depletion and high chondrite normalized Gd/Yb ratios. These basalts are interpreted to have erupted in the age of 157.0 ± 0.9 Ma (Late Jurassic), based on zircon U–Pb dating of anorthosite. The basalt geochemistry indicates that oceanic plateau basalts occur in the Kashiwagi Unit. Although basalts in the Mikabu Unit were thought to have formed at the Pacific–Izanagi–Farallon triple junction in the previous studies, we propose they formed on older oceanic crust of the Izanagi Plate during the Late Jurassic, which was located several thousand kilometers from the triple junction. The Kashiwagi and Mikabu units were accreted to Asia during the Early Cretaceous.

Paleoproterozoic metamorphism in the northern Fennoscandian Shield: age constraints revealed by monazite

ABSTRACTWe have identified two contrasting styles of Paleoproterozoic metamorphism in the northern part of the Fennoscandian Shield. The Karelia and Lapland-Kola Provinces, comprising Archean and overlying Paleoproterozoic supracrustal rocks, show a typical medium pressure Barrovian-style metamorphism with commonly found kyanite-bearing mineral assemblages and ITD (isothermal decompression) PT paths. In the juxtaposed Svecofennia Province metamorphism represents low pressure-high temperature Buchan style with garnet-cordierite migmatites and intercalated andalusite-cordierite and andalusite-staurolite schists and sillimanite-muscovite gneisses. The retrograde PT paths show only a moderate uplift during cooling.U-Pb age determinations on monazite were made using the LA-ICP-MS from more than 80 samples from metasedimentary rocks. The sampling covered most parts of the Paleoproterozoic bedrock in Finland. The analyses reveal three peaks at c. 1.91 Ga, 1.86–1.88 Ga and at 1.79–1.81 Ga. The oldest, c. 1.91 Ga monazites are mostly found in the Lapland-Kola Province which is located in the northernmost Finland. In the Karelia Province where the Paleoproterozoic is underlain by Archean bedrock monazite yielded ages of 1.76‒1.81 Ga with only a few older exceptions in samples showing a spread of 207Pb/206Pb ages from c. 1.92–1.81 Ga. The Karelia Province underwent tectonic thickening, where monazite ages of around 1.80 Ga mostly represent exhumation near the temperature maximum.In the Svecofennia Province monazite ages vary from c. 1.89 to 1.78 Ga. In the Western Finland Subprovince the monazite ages in high-grade migmatites are mostly 1.86‒1.88 Ga but within the older migmatite areas there are lower grade zones where monazite yields ages of c. 1.80 Ga. Some samples also show a spread of 207Pb/206Pb ages from 1.89‒1.86 Ga to c. 1.78 Ga. In the Southern Finland Subprovince most ages are either 1.80‒1.78 Ga, especially in the andalusite grade schists, or the sample shows a spread of 207Pb/206Pb ages from c. 1.88 to 1.78 Ga. Only in the eastern part of the Southern Finland Subprovince there are rocks which yield merely 1.86‒1.89 Ga ages. Low pressure-high temperature metamorphism and lack of high or medium P/T rocks in the Svecofennia Province refers rather to accretionary than collisional processes.

First evidence of Archean mafic dykes at 2.62 Ga in the Yilgarn Craton, Western Australia: Links to cratonisation and the Zimbabwe Craton

The Archean Yilgarn Craton in Western Australia hosts at least five generations of Proterozoic mafic dykes, the oldest previously identified dykes belonging to the ca. 2408–2401 Ma Widgiemooltha Supersuite. We report here the first known Archean mafic dyke dated at 2615 ± 6 Ma by the ID-TIMS U-Pb method on baddeleyite and at 2610 ± 25 Ma using in situ SHRIMP U-Pb dating of baddeleyite. Aeromagnetic data suggest that the dyke is part of a series of NE-trending intrusions that potentially extend hundreds of kilometres in the southwestern part of the craton, here named the Yandinilling dyke swarm. Mafic magmatism at 2615 Ma was possibly related to delamination of the lower crust during the final stages of assembly and cratonisation, and was coeval with the formation of late-stage gold deposit at Boddington. Paleogeographic reconstructions suggest that the Yilgarn and Zimbabwe cratons may have been neighbours from ca. 2690 Ma to 2401 Ma and if the Zimbabwe and Kaapvaal cratons amalgamated at 2660–2610 Ma, the 2615 Ma mafic magmatism in the southwestern Yilgarn Craton may be associated with the same tectonic event that produced the ca. 2607–2604 Ma Stockford dykes in the Central Zone of the Limpopo Belt. Paleomagnetic evidence and a similar tectonothermal evolution, including coeval low-pressure high-temperature metamorphism, voluminous magmatism, and emplacement of mafic dykes, support a configuration where the northern part of the Zimbabwe Craton was adjacent to the western margin of the Yilgarn Craton during the Neoarchean. Worldwide, reliably dated mafic dykes of this age have so far been reported from the Yilgarn Craton, the Limpopo Belt and the São Francisco Craton.

New palaeobotanical data from Carboniferous Culm deposits constrain the age of the Variscan deformation in the eastern Pyrenees

The pre-Variscan rocks of the Pyrenees exhibit a polyphase deformation linked to the Variscan crustal shortening and a low-pressure–high-temperature metamorphism. However, there is scarce chronostratigraphic evidence of this Variscan deformation. In the Pyrenean low-grade metamorphic domains, maximum ages have been provided by the synorogenic Carboniferous Culm deposits. In medium- to high-grade metamorphic areas, the Variscan regional metamorphism or intrusive magmatic bodies constrain the age of the main Variscan deformation structures. However, these data usually provide a minimum age. Here, we present new palaeobotanical records that assign a Namurian age to the base of the Culm deposits of la Cerdanya in the eastern Pyrenees. This dating is based on the co-occurrence of the sphenopsids Archaeocalamites radiatus , Mesocalamites cistiiformis and the seed of Cardiocarpus sp. The plant remains were found in sandstone facies produced by high-density turbidity flows of a deep-sea fan system. The new biostratigraphic information constrains the age of the Carboniferous Culm succession in the eastern Pyrenees.

High-temperature–low-pressure metamorphism and the production of S-type granites of the Hillgrove Supersuite, southern New England Orogen, NSW, Australia

ABSTRACTThe high-temperature–low-pressure Wongwibinda Metamorphic Complex of the southern New England Orogen is bound by S-type granite plutons of the Hillgrove Supersuite to the north, east and south. New U–Pb geochronology of five samples of the Hillgrove Supersuite demonstrates that plutonism in the complex involved two pulses: ca 300 Ma and ca 292 Ma. This indicates that plutonism partially overlaps the age of high-T–low-P metamorphism (296.8 ± 1.5 Ma), but also postdates it. Zircon grains identified as xenocrysts based on age (≥310 Ma) have U–Pb–Hf isotopic character that largely overlaps detrital grains in the host Girrakool Beds, indicating that accretionary complex crust is the likely source of these xenocrysts. The 176Hf/177Hf initial character for zircon for the ca 300 Ma plutons (three samples) is less radiogenic than those in the ca 292 Ma plutons (two samples). The progression in 176Hf/177Hf initial character for zircon infers an increasing mantle component in the Hillgrove Supersuite with time. These data are evidence of a rift tectonic setting, where mantle-derived magmas are predicted to more readily migrate to shallower crustal levels as the crust thins and becomes hotter. Additionally, early episodes of partial melting in the system melt-depleted the metasedimentary sources, thus reducing the S-type component as anatexis progressed. The evolution of the Hillgrove Supersuite coincides with a period of early Permian slab roll back and extension accompanied by crustal rifting and thinning, leading to high-T–low-P metamorphism, anatexis and S-type granite production and the development of rift basins such as the Sydney–Gunnedah–Bowen system.

Prolonged high-temperature, low-pressure metamorphism associated with ∼1.86 Ga Sancheong–Hadong anorthosite in the Yeongnam Massif, Korea: Paleoproterozoic hot orogenesis in the North China Craton

Elevated heat flow in association with mafic magmatism in an orogenic belt commonly leads to high-temperature, low-pressure (HTLP) metamorphism and the production of granulite-facies assemblages. We studied such a HTLP complex in the vicinity of the Sancheong–Hadong anorthosite-mangerite-charnockite-granite (AMCG) suite, Yeongnam Massif, Korea, in order to constrain the P–T conditions, timing, and duration of metamorphism. This complex primarily consists of massif-type anorthositic-gabbroic bodies emplaced at ∼1.87–1.86 Ga and a series of country rocks comprising orthopyroxene-bearing gneisses as well as anatectic granites and migmatites. Migmatitic gneisses were studied in detail because melt-related features are abundant and well preserved; for example, inclusion-rich peritectic phases such as cordierite or K-feldspar are characteristic for the prograde melt-forming stage, whereas biotite-quartz symplectites mantling garnet or orthopyroxene represent the cooling stage consuming melt. Pseudosection P–T analyses of migmatitic gneisses suggest peak metamorphic conditions of 810–840 °C and 5.9–6.2 kbar, followed by near-isobaric cooling and melt crystallization at ∼780 °C and ∼5.5 kbar. SHRIMP (sensitive high-resolution ion microprobe) U–Th–Pb ages of zircon and monazite from six migmatitic gneisses are in the range of 1870–1854 Ma. The oldest age, recorded only in high Y monazite, suggests that prograde metamorphism commenced at ∼1870 Ma. In contrast, melt crystallization had culminated by 1860–1855 Ma producing widespread leucosomes and anatectic granites. Our results suggest that high thermal gradient (∼40 °C km−1) attending the granulite-facies metamorphism is attributable to coeval, pulse-like emplacement of anorthositic-gabbroic magmas. Moreover, the HTLP metamorphism lasted over a period of ∼15 Ma, indicating a long-lived process corresponding to the late stage of Paleoproterozoic (∼1.95–1.85 Ga) hot orogenesis in the North China Craton.

Geochemistry and petrogenesis of Kolah-Ghazi granitoids of Iran: Insights into the Jurassic Sanandaj-Sirjan magmatic arc

Kolah-Ghazi granitoid (KGG), situated in the southern part of the Sanandaj–Sirjan Zone (SNSZ), Iran, is a peraluminous, high K calc-alkaline, cordierite-bearing S-type body that is mainly composed of monzogranite, granodorite and syenogranite. Zircon U–Pb ages indicate that the crystallization of the main body occurred from 175 Ma to 167 Ma. Two kinds of xenoliths are found in KKG rocks: (i) xenoliths of partially melted pelites including cordierite xenocrysts and aluminoslicates, and (ii) mafic microgranular enclaves that reflect the input of mantle-derived mafic magmas. Field observations and geochemical data of KGG rocks are consistent with their derivation from a multiple sources including melts of metasediments and mantle-derived melts. We infer that these magmas originated by the anatexis of a metasedimentary source (mixture of metapelite and metagreywacke) in the mid- to lower-crust under low water-vapor pressures (0.5-1 Kbar) and temperature of ∼800°C. KGG is the product of biotite incongruent melting of this metasedimentary source. S-type granites are commonly thought to be produced in continent-continent collision tectonic environment. However, trace element discrimination diagrams show that S-type KGG rocks formed in an arc-related environment. The roll-back of Neo- Tethyan subducting slab accompanying oblique subduction in Late Triassic to Early Jurassic time induced trench rollback, back arc basin opening and filling with turbidite flysch and molasse- type siliciclastic sediments of the Shemshak Group on the overriding plate. Further changes in the subducting slab to flat subduction in Middle Jurassic time, the time of peak magmatism in the SNSZ, led to thickening and high temperature-low pressure metamorphism of the backarc turbidite deposits and consequent anatexis of the metasedimentary source to produce the KGG S- type rocks along with several other I-type granitoids in the SNSZ.

The Keepit arc: provenance of sedimentary rocks in the central Tablelands Complex, southern New England Orogen, Australia, as recorded by detrital zircon

ABSTRACTDetrital zircon from the Carboniferous Girrakool Beds in the central Tablelands Complex of the southern New England Orogen, Australia, is dominated by ca 350–320 Ma grains with a peak at ca 330 Ma; there are very few Proterozoic or Archean grains. A maximum deposition age for the Girrakool Beds of ca 309 Ma is identified. These data overlap the age of the Carboniferous Keepit arc, a continental volcanic arc along the western margin of the Tamworth Belt. Zircon trace-element and isotopic compositions support petrographic evidence of a volcanic arc provenance for sedimentary and metasedimentary rocks of the central Tablelands Complex. Zircon Hf isotope data for ca 350–320 Ma detrital grains become less radiogenic over the 30 million-year record. This pattern is observed with maturation of continental volcanic arcs but is opposite to the longer-term pattern documented in extensional accretionary orogens, such as the New England Orogen. Volcanic activity in the Keepit arc is inferred to decrease rapidly at ca 320 Ma, based on a major change in the detrital zircon age distribution. Although subduction continues, this decrease is inferred to coincide with the onset of trench retreat, slab rollback and the eastward migration of the magmatic arc that led to the Late Carboniferous to early Permian period of extension, S-type granite production and intrusion into the forearc basin, high-temperature–low-pressure metamorphism, and development of rift basins such as the Sydney–Gunnedah–Bowen system.

Thermal history and extensional exhumation of a high-temperature crystalline complex (Hırkadağ Massif, Central Anatolia)

The Central Anatolian Crystalline Complex (CACC) is a large continental domain exposed in central Turkey that was affected by high temperature metamorphism during the Late Cretaceous. As a result of this event, Paleozoic sediments became metamorphosed, initially under Barrovian conditions, then overprinted locally by high temperature–low pressure metamorphism, and intruded by widespread batholiths. In this study we focus on the crystalline Hırkadağ Massif located in the central part of the CACC, where we applied an integrated approach involving metamorphic, structural and geochronological analysis in order to elucidate its tectonic history from burial to exhumation. Our metamorphic study reveals that conditions of metamorphism reached ~7–8kbar/700°C and were relatively homogeneous at the scale of the Hırkadağ Massif. Coeval with the regional metamorphism, the rocks were intensely deformed as reflected by isoclinal folding, the development of a pervasive foliation and top-to-the-SE shearing. This was followed by decompression to pressures of ~3–4kbar at 800°C, which may be linked to the emplacement of local granodioritic intrusions at ~77Ma. Subsequent cooling of the Hırkadağ high-grade metamorphic and intrusive rocks is indicated by 40Ar/39Ar cooling ages of 68.8±0.9Ma (biotite) and 67.0±1.2Ma (potassium feldspar). Evidence for tectonic exhumation has been identified within the marbles at the NE margin of the Hırkadağ Massif, in the form of discrete protomylonitic and mylonitic shear bands showing a consistent N40–60 top-to-NE sense of shear. Further east, the contact between brecciated mylonitic marbles and non-metamorphic conglomerates preserves the typical structural features of an upper-crustal detachment fault. Restoration of the Hırkadağ Massif and the CACC to their late Cretaceous configuration suggests that the LP–HT metamorphism, magmatism and extensional structures evolved as a result of the development and exhumation of a ~N–S trending magmatic arc experiencing regional E–W extension above an active subduction zone.

High-temperature metamorphism during extreme thinning of the continental crust: a reappraisal of the North Pyrenean passive paleomargin

Abstract. An increasing number of field examples in mountain belts show that the formation of passive margins during extreme continent thinning may occur under conditions of high to very high thermal gradient beneath a thin cover of syn-rift sediments. Orogenic belts resulting from the tectonic inversion of distal margins and regions of exhumed continental mantle may exhibit high-temperature, low-pressure (HT-LP) metamorphism and coeval syn-extensional, ductile deformation. Recent studies have shown that the northern flank of the Pyrenean belt, especially the North Pyrenean Zone, is one of the best examples of such inverted hot, passive margin. In this study, we provide a map of HT-LP metamorphism based on a data set of more than 100 peak-temperature estimates obtained using Raman spectroscopy of the carbonaceous material (RSCM). This data set is completed by previous PT (pressure and temperature) estimates based on mineral assemblages, and new 40Ar–39Ar (amphibole, micas) and U–Pb (titanite) ages from metamorphic and magmatic rocks of the North Pyrenean Zone. The implications on the geological evolution of the Cretaceous Pyrenean paleomargins are discussed. Ages range mainly from 110 to 90 Ma, and no westward or eastward propagation of the metamorphism and magmatism can be clearly identified. In contrast, the new data reveal a progressive propagation of the thermal anomaly from the base to the surface of the continental crust. Focusing on the key localities of the Mauléon basin, Arguenos–Moncaup, Lherz, Boucheville and the Bas-Agly, we analyze the thermal conditions prevailing during the Cretaceous crustal thinning. The results are synthetized into a series of three regional thematic maps and into two detailed maps of the Arguenos–Moncaup and Lherz areas. The results indicate a first-order control of the thermal gradient by the intensity of crustal thinning. The highest grades of metamorphism are intimately associated with the areas where subcontinental mantle rocks have been unroofed or exhumed.

40Ar–39Ar biotite age of a lamprophyre dyke and constraints on the timing of ductile deformation inside the Idefjorden terrane and along the Mylonite Zone, Sveconorwegian orogen, south-west Sweden

A sharply cross-cutting lamprophyre dyke inside the Idefjorden terrane and along the Mylonite Zone in the Sveconorwegian orogen, Sweden, yields a plateau 40Ar–39Ar biotite age of 914.6 ± 1.2 Ma. This result confirms a published K–Ar age and is interpreted to record magmatic intrusion of the dykes. The lamprophyres are coeval to the similarly oriented but less potassic 916 ± 11 Ma noritic to anorthositic Hakefjorden Complex exposed west of the Göta Älv Shear Zone in the Idefjorden terrane. The lamprophyres are characterized by high magnetic susceptibility, chilled margins, biotite phenocrysts and deuteric alteration of the groundmass. They classify as kersantite and minette, and are interpreted to represent rare, minor explosive mantle-derived magmas, intruded along deep-reaching, east-west trending, extensional structures related to the waning stages of the Sveconorwegian orogeny. Since the host bedrock, at the current level of erosion, was responding to brittle deformation prior to the intrusion of the lamprophyre dykes, the age of 914.6 ± 1.2 Ma sets a minimum limit for the termination of Sveconorwegian ductile deformation inside the Idefjorden terrane and along the Mylonite Zone, north of Lake Vänern. This contrasts to the situation in the westernmost Rogaland–Vest Agder sector of the Sveconorwegian orogen, in Norway, where 930–920 Ma anorthosite to granite plutonism was spatially related to high temperature-low pressure metamorphism and ductile deformation during the waning stages of the Sveconorwegian orogeny. The new age underscores the diachronic exhumation of the orogen.

Garnet oxygen analysis by SHRIMP-SI: Matrix corrections and application to high-pressure metasomatic rocks from Alpine Corsica

Garnet is a key mineral used to constrain pressure, temperature and age of metamorphic rocks. This contribution reports oxygen isotope measurements in garnet using the SHRIMP-SI ion microprobe. The reproducibility of oxygen isotope analyses on garnet standard UWG2 is ~0.3–0.4‰ (2σ) within and across sessions. The correlation between oxygen isotope measurements and the grossular and andradite components in garnet fits a second-degree polynomial with a maximum bias in δ18O of 2.4 and 8.3‰, respectively. This bias is similar to that determined for other large ion microprobes. Analysis of two additional Mn-rich garnet crystals allowed identification of a separate bias caused by the spessartine component, which can reach a maximum of 2.3‰.The standardisation and correction scheme proposed in this study are applied to garnet crystals from two samples from Alpine Corsica in order to link fluid evolution with the pressure–temperature–time path. The samples have experienced a polyphase metamorphic history, which includes Permian high temperature metamorphism, followed by late However, 92-W2 was re-analysed because of the variability observed in the previously published δ18O values (-0.29 and 0.53‰, Kohn and Valley, 1998). The δ18O value found in this study (0.81 ± 0.44‰, 2s, Table 1) agrees within error with the highest value previously published for this garnet. Eocene high pressure–low temperature metamorphism. Permian Ca-poor garnet cores have high δ18O values (9.9±0.6‰ and 11.1±0.5‰ 2σ, in two samples) with respect to garnet mantles (7.2±0.4‰) and rims (5.4±0.5‰ and 2.2±0.4‰). The dramatic decrease in δ18O from Permian garnet cores to Alpine rims in both samples reflects a combination of external fluid influx and change in sample mineralogy. The low δ18O of the garnet rims that formed at eclogite facies conditions indicates that the metasomatic fluid equilibrated with mafic or ultramafic rocks. This study illustrates that fluid-mediated mass transfer during subduction occurred at lithological contacts between felsic and ultramafic rocks.