Garnet Amphibolite Research Articles

Two episodes of metamorphism in the rocks of the eastern Kitoi block (Sharyzhalgai uplift of the Siberian platform) according to the garnet amphibolite data

The Archean metamorphism in the southwestern Siberian craton was confirmed by the studies of the Kitoy and Irkutsk blocks. However, the parameters of the metamorphism process are still poorly investigated. The article presents the first results of studying the metamorphism conditions of garnet amphibolites sampled from the Kitoy sillimanite deposit. The reaction relationships of the studied minerals give grounds to distinguish two episodes of the regional metamorphism. At the end of the first episode, (Т=710–770 °С and Р=8.3–8.8 kb), the pressure reduced to 1.3–2.5 kbar at T=700 °C at the retrograde stage, and amphibole-plagioclase rims formed around garnet grains. During the second episode of metamorphism, the temperature reached 890 °С (granulite facies), and Cpx+Opx paragenesis replaced hornblende. The second episode of metamorphism is not evident in all the samples (considering the same bulk rocks composition of the rocks), which suggests its local character.

Mineralogy and geothermo-barometry of metapelitic schists, amphibolites and garnet amphibolites from Gol-Gohar metamorphic complex, SW Sirjan, Central Iran.

Mineralogy and geothermo-barometry of metapelitic schists, amphibolites and garnet amphibolites from Gol-Gohar metamorphic complex, SW Sirjan, Central Iran.

Diverse subduction and exhumation of tectono-metamorphic slices in the Kalatashitage area, western Paleozoic Dunhuang Orogenic Belt, northwestern China

High-pressure mafic granulite and garnet amphibolite are identified as small-scale tectonic slices within pelitic or semi-pelitic gneiss in the Kalatashitage area, which is located in the western Paleozoic Dunhuang Orogenic Belt, northwestern China. These rocks retain three generations of metamorphic mineral assemblages: prograde assemblage (M1) preserved as inclusions within garnet porphyroblasts, metamorphic peak assemblage (M2) consisting of matrix minerals and garnet porphyroblasts, and retrograde assemblage (M3) mainly represented by the symplectic minerals surrounding the embayed garnet and the retrograded hornblende rimming matrix-type clinopyroxene. Metamorphic pressure and temperature (P-T) paths of high-pressure mafic granulite, amphibolite, and metapelite retrieved by thermobarometry are all clockwise, passing from 640 to 720 °C/6.2–12.6 kbar (M1) through 840–920 °C/14.6–16.2 kbar (M2) to 750–815 °C/5.5–7.9 kbar (M3) for high-pressure mafic granulite, from ~650 °C/5.7 kbar (M1) through ~750 °C/9.2 kbar (M2) to ~780 °C/8.1 kbar (M3) for amphibolite, and from ~615 °C/7.9 kbar (M1) through 730–820 °C/8.6–11.7 kbar (M2) to 675–740 °C/5.4–8.7 kbar (M3) for pelitic and semi-pelitic gneiss. Furthermore, pseudosection modeling of high-pressure mafic granulite indicates that the growth zonation of garnet porphyroblast exhibits prograde metamorphism in a P-T range of 510–800 °C/8.5–13 kbar and demonstrates peak metamorphic P-T conditions of ~850 °C/16 kbar, which are consistent with the thermobarometric estimates. The significant pressure differences in peak metamorphism observed in different rocks indicate that the rocks initially subducted to remarkably different depths and were subsequently juxtaposed at shallower crustal levels during exhumation. Sensitive high-resolution ion microprobe (SHRIMP) analysis and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) UPb dating of metamorphic zircon indicates that the metamorphic events occurred at ca. 430–420 Ma (M2) and ca. 400–390 Ma (M3), respectively. Metamorphism was followed by the intrusion of granitic dykes at ca. 244 Ma. Moreover, the metamorphic evolution indicates that the Kalatashitage area was involved in the subduction, collision and subsequent tectonic exhumation in the Paleozoic. Combined with previous literature, it is inferred that the discrepant subduction and exhumation of high-grade metamorphic rocks is a universal phenomenon in the Paleozoic Dunhuang Orogenic Belt, supporting the ubiquitous existence of subduction-collision complexes in this orogenic belt.

Metamorphic evolution and 40Ar/39Ar geochronology of the Wuguan complex, eastern Qinling area, China: Implications for the late Paleozoic tectonic evolution of the Qinling orogen

The late Paleozoic tectonic evolution of the Qinling orogen is poorly constrained. The Wuguan Complex, which lies between the North Qinling and South Qinling tectonic belts, represents a late Paleozoic metamorphic belt of the Qinling orogen. In this contribution, we report new metamorphic pressure–temperature (P–T) constraints and 40Ar/39Ar ages of garnet amphibolites and garnet-bearing metapelites from the Wuguan Complex. Textural observations and mineral compositions, especially garnets with two stages of growth, indicate the Wuguan Complex experienced two distinct metamorphic events. Conventional geothermobarometry and phase equilibria modelling indicate that the earlier metamorphism had peak P–T conditions of 660–725 °C and 8.0–12.0 kbar with a clockwise P–T path, whereas the later metamorphic overprint records P–T conditions of 560–610 °C and 7.5–9.0 kbar. Hornblendes from amphibolites yield 40Ar/39Ar ages of ~306–299 Ma, whereas muscovites from garnet-bearing metapelites yield 40Ar/39Ar ages of ~252–251 Ma. The two groups of 40Ar/39Ar ages correspond to the cooling time of the two distinct metamorphic events, respectively. Combining with previously published zircon UPb ages from the Wuguan Complex and two groups of metamorphic ages from the Guishan Complex and the Xiongdian eclogite belt, we infer that the earlier metamorphism probably occurred at the hangingwall of a subduction zone during Carboniferous subduction of oceanic crust. The later metamorphic overprint might have been related to the initial collision between the Sino-Korean and Yangtze cratons during the late Permain.

Generation and maturation of Mesoarchean continental crust in the Anshan Complex, North China Craton

The mechanisms underlying the generation of tonalite–trondhjemite–granodiorite suites and the transition to K-rich granitoids, which marks the most profound stage of Archean intracrustal differentiation, remain controversial. The Archean Anshan Complex (AAC) of the North China Craton (NCC) preserves diverse 3.8–2.5 Ga granitoids, providing an excellent opportunity to address these problems. We conducted a detailed field, petrological, geochemical, and zircon isotopic study on AAC units, demonstrating that: (1) the Li–Dong–Ying trondhjemite intrudes and encloses ca. 3.3 Ga gneiss–migmatite complex across the Anshan, and crystallized at ca. 3.14 Ga; and (2) the porphyritic and fine-grained Chentaigou monzogranites were emplaced contemporaneously with the trondhjemite, both at ca. 3.14 Ga.The Li–Dong–Ying trondhjemites show high SiO2, Na2O, low MgO, significant variations in Sr, Y, and Yb, REE fractionation (LaN/YbN = 9–140), and variable Eu anomalies (Eu/Eu* = 0.4–1.3). The magmatic zircons yield δ18O of +4.8 ± 0.2‰ to +6.9 ± 0.4‰ and initial εHf of −1.1 ± 0.7 to +4.4 ± 0.9, which translate to TDM ages of 3.6–3.3 Ga. The Chentaigou monzogranites have higher SiO2 and K2O, lower Na2O and CaO, similar δ18O of +5.1 ± 0.3‰ to +7.0 ± 0.3‰, and more negative Eu anomalies as compared with the trondhjemites. The Hf–O isotopes, geochemistry, particularly considerable variations in pressure-sensitive proxies of the trondhjemites, suggest that they were derived by melting of low-temperature altered basaltic rocks at 1.0–1.5 GPa and 900–950 °C, leaving a garnet amphibolite residue with variable amounts of plagioclase but no rutile; the most plausible source is ca. 3.3 Ga basaltic rocks. Polybaric melting of basaltic sources and fractionation of hornblende and plagioclase from trondhjemitic magmas account for the chemical variability of AAC granitoids, providing a mechanism for the coeval emplacement of diverse Archean granitoids. Secular compositional changes in AAC trondhjemites imply that the depth and interval of melting of basaltic sources increased from the Eoarchean to Mesoarchean. Changes in the nature of the source and melting depth characterized the early stage of Archean intracrustal differentiation in the NCC.

Pressure-temperature conditions and significance of Upper Devonian eclogite and amphibolite facies metamorphisms in southern French Massif central

The southwestern French Massif central in western Rouergue displays an inverted metamorphic sequence with eclogite and amphibolite facies units forming the top of the nappe stack. They are often grouped into the leptyno-amphibolite complex included, in this area, at the base of the Upper Gneiss Unit. We sampled garnet micaschists and amphibolites to investigate their metamorphic history with isochemical phase diagrams, thermobarometry and U-Pb zircon dating. Our results demonstrate that two different tectono-metamorphic units can be distinguished. The Najac unit consists of biotite-poor phengite-garnet micaschists, a basic-ultrabasic intrusion containing retrogressed eclogites and phengite orthogneisses. Pressure and temperature estimates on micaschists with syn-kinematic garnets yield a prograde with garnet growth starting at 380 °C/6–7 kbar, peak pressure at 16 kbar for 570 °C, followed by retrogression in the greenschist facies. The age of high pressure metamorphism has been constrained in a recent publication between ca. 383 and 369 Ma. The Laguépie unit comprises garnet-free and garnet-bearing amphibolites with isolated lenses, veins or dykes of leucotonalitic gneiss. Thermobarometry and phase diagram calculation on a garnet amphibolite yield suprasolidus peak P-T conditions at 710 °C, 10 kbar followed by retrogression and deformation under greenschist and amphibolite facies conditions. New U-Pb analyses obtained on igneous zircon rims from a leucotonalitic gneiss yield an age of 363 ± 3 Ma, interpreted as the timing of zircon crystallization after incipient partial melting of the host amphibolite. The eclogitic Najac unit records the subduction of a continental margin during Upper Devonian. It is tentatively correlated to a Middle Allochthon, sandwiched between the Lower Gneiss Unit and the Upper Gneiss Unit. Such an intermediate unit is still poorly defined in the French Massif central but it can be a lateral equivalent of the Groix blueschists in the south Armorican massif. The Uppermost Devonian, amphibolite facies Laguépie unit correlates in terms of P-T-t evolution to the Upper Gneiss Unit in the Western French Massif central. This Late Devonian metamorphism is contemporaneous with active margin magmatism and confirms that the French Massif central belonged to the continental upper plate of an ocean-continent subduction system just before the stacking of Mississippian nappes.

Reconstruction the Process of Partial Melting of the Retrograde Eclogite from the North Qaidam, Western China: Constraints from Titanite U-Pb Dating and Mineral Chemistry

Retrograde eclogite and garnet amphibolite of the Luliangshan unit of the Shenglikou area, North Qaidam, were studied with emphasis on rutile and titanite. A special focus is on the formation of rutile and its corona of titanite (Ttn1) in retrograde eclogite and on coarse-grained titanite (Ttn2) from the garnet amphibolite. Using zirconium (Zr)-in-rutile and Zr-in-titanite thermometers, the temperatures estimated for the formation of an early generation of rutile are 823–884 °C at 2.5–2.8 GPa, while 812–894 °C at 1.3–1.5 GPa are derived for the formation of coronitic Ttn1 in the retrograde eclogite. Therefore, isothermal decompression must have occurred during exhumation, which also has triggered the partial melting of the retrograde eclogite. Ttn2 of the garnet amphibolite has high REE contents and high Th/U ratios, indicating that it is newly grown from a Ti, Ca, and LREE enriched anatectic melt derived from the partial melting of retrograde eclogite. LA-ICP MS U-Pb dating yields a lower intercept age of 423±4 Ma for Ttn2, which is consistent with the granulite-facies metamorphic age of the retrograde eclogite. Moreover, a temperature of 781–823 °C at 1.0–1.2 GPa is obtained for Ttn2, which fits the P-T conditions of the HP granulite-facies metamorphic stage (P=1.07–1.24 GPa and T=774–814 °C), and documents that the crystallization of the melt occurred at the granulite-facies stage at 423 Ma. The high amount of REE of the garnet amphibolite is a consequence of the formation of Ttn2 from the melt. The contents and ratios of Zr and Hf in rutile and Ttn2 differ from those in the garnet amphibolite, and the whole rock Zr/Hf ratios of retrograde eclogite and garnet amphibolite are both higher than the respective ratios in rutile and Ttn2, suggesting that rutile and titanite cannot be the major carriers of Zr and Hf accounting for the high whole rock Zr/Hf ratios.

Adakite‐Like Potassic Magmatism and Crust‐Mantle Interaction in a Postcollisional Setting: An Experimental Study of Melting Beneath the Tibetan Plateau

AbstractAdakite‐like potassic rocks are widespread in postcollisional settings where they provide potential insights into deep crustal processes that include partial melting, lower crustal flow and thickening, plateau uplift, and the creation of porphyry metal deposits. Although substantial progress has been made in characterizing the geochemical and geophysical features of postcollisional adakite‐like potassic rocks, their petrogenesis remains controversial. Here we report direct experimental evidence for the origins of these rocks with partial melting experiments on (1) garnet amphibolite and (2) the same garnet amphibolite mixed with 20 wt. % of a primitive Tibetan shoshonite. The experiments were conducted at 1.5–2.0 GPa and 800–1,000 °C. The partial melts of garnet amphibolite have typical adakitic signatures and are calc‐alkalic but lack the enrichment in potassium and other strongly incompatible elements (Rb, Ba, Th, U) that are characteristic of Tibetan adakite‐like rocks. In contrast, all characteristic features of the natural adakite‐like rocks are convincingly reproduced by the hybrid experiments. This includes negative inflections for Nb, Ta, and Ti on mantle‐normalized plots which are inherited from source materials rather than the effects of rutile in melting residues. The input of mantle‐derived shoshonitic mafic melts to a crustal source can be argued to provide not only the high concentrations of incompatible elements characteristic of adakite‐like potassic magmas but also the heat necessary for crustal melting. Our experimental results demonstrate that, in the case of the Tibetan Plateau at least, the production of adakite‐like potassic rocks in postcollisional settings can be best explained by such a model.

Timanide (Ediacaran-Early Cambrian) Metamorphism at the Transition from Eclogite to Amphibolite Facies in the Beloretsk Complex, SW-Urals, Russia

The Beloretsk Metamorphic Complex in the SW Urals formed at a convergent eastern margin of Baltica during the Neoproterozoic-Early Cambrian Timanide orogeny. It comprises three major units with lenses of facies-critical metabasites within metasedimentary rocks: A lowermost eclogite unit, an intermediate garnet amphibolite unit and an upper amphibolite-greenschist unit. Pressure (P)-temperature (T)-paths of four rocks from the two lowermost units were determined mainly by PT pseudosection techniques showing similar clockwise loops at different peak metamorphic, water-saturated conditions: A phengite-bearing eclogite shows peak PT conditions of 16.5–18.5 kbar/525–550 °C (stage I) followed by stage II at 11.5–13.0 kbar/585–615 °C. A garnet amphibolite from the intermediate unit yields lower peak conditions of 11.7–14.5 kbar/480–510 °C (stage I) followed by stage II at 9.5–11.0 kbar/535–560 °C. However, a granite gneiss in the eclogite unit shows similar maximum pressures as the eclogite, but higher temperatures at 15.6–16.2 kbar/660–675 °C, whereas a garnet micaschist contains comparable high pressure relicts, but underwent an advanced midcrustal reequilibration at 7.5–9.0 kbar/555–610 °C. We dated the eclogite by a 7-point Rb/Sr mineral isochron (phengite, omphacite, apatite) at 532.2±9.1 Ma interpreted as age of crystallisation of the eclogitic peak PT assemblage. This age is the youngest compared to the known Timanide metamorphic and magmatic ages.

Metamorphism and geochronology of garnet amphibolite from the Beishan Orogen, southern Central Asian Orogenic Belt: Constraints from P-T path and zircon U-Pb dating

Numerous lenses of garnet amphibolite occur in the garnet-bearing biotite-plagioclase gneiss belt in the Baishan area of the Beishan Orogen, which connects the Tianshan Orogen to the west and the Mongolia-Xing’anling Orogen to the east. The study of metamorphism in Beishan area is of great significance to explain the tectonic evolution of Beishan orogen. According to the microstructures, mineral relationships, and geothermobarometry, we identified four stages of mineral assemblages from the garnet amphibolite sample: (1) a pre-peak stage, which is recorded by the cores of garnet together with core-inclusions of plagioclase (Pl1); (2) a peak stage, which is recorded by the mantles of garnet together with mantle-inclusions of plagioclase (Pl2) ​+ ​amphibole (Amp1) ​+ ​Ilmenite (Ilm1) ​+ ​biotite (Bt1), developed at temperature-pressure (P-T) conditions of 818.9–836.5 ​°C and 7.3–9.2 ​kbar; (3) a retrograde stage, which is recorded by garnet rims ​+ ​plagioclase (Pl3) ​+ ​amphibole (Amp2) ​+ ​orthopyroxene (Opx1) ​+ ​biotite (Bt2) ​+ ​Ilmenite (Ilm2), developed at P-T conditions of 796.1–836.9 ​°C and 5.6–7.5 ​kbar; (4) a symplectitic stage, which is recorded by plagioclase (Pl4) ​+ ​orthopyroxene (Opx2) ​+ ​amphibole (Amp3) ​+ ​biotite (Bt3) symplectites, developed at P-T conditions of 732 ​± ​59.6 ​°C and 6.1 ​± ​0.6 ​kbar. Moreover, the U-Pb dating of the Beishan garnet amphibolite indicates an age of 301.9 ​± ​4.7 ​Ma for the protolith and 281.4 ​± ​8.5 ​Ma for the peak metamorphic age. Therefore, the mineral assemblage, P-T conditions, and zircon U-Pb ages of the Beishan garnet amphibolite define a near-isothermal decompression of a clockwise P-T-t (Pressure-Temperature-time) path, indicating the presence of over thickened continental crust in the Huaniushan arc until the Early Permian, then the southern Beishan area underwent a process of thinning of the continental crust.

Two phases of post-onset collision adakitic magmatism in the southern Lhasa subterrane, Tibet, and their tectonic implications

AbstractAbundant Neogene adakitic magmatism occurred in the southern Lhasa subterrane after the onset of the India–Asia collision while convergence continued. However, the tectonic setting and magmatic evolution of the adakitic rocks are still under discussion. This study includes new mineral chemical and whole-rock geochemical data as well as zircon U-Pb and Lu-Hf isotopes of adakitic intrusive rocks from the Gyaca and Nyemo locations in the southern Lhasa subterrane. Laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) zircon U-Pb dating yielded crystallization ages of ca. 30 Ma for the Gyaca and Nyemo monzogranite and ca. 15 Ma for the Nyemo granodiorite. Both have common chemical signatures such as low MgO and heavy rare earth element contents as well as low compatible element abundances, indicating that these rocks result from partial melting of thickened lower crust with residual eclogite and garnet amphibolite. Furthermore, these rocks are characterized by variable positive zircon εHf(t) values, suggesting a juvenile magma source with variable ancient crustal contributions. Taking previous data into account, the adakitic magmatism concurs with an early late Eocene to Oligocene (ca. 38–25 Ma) and a late Miocene (ca. 20–10 Ma) phase. The adakitic rocks of the two phases are characterized by different fractionation evolutions of light and medium rare earth elements. We propose that the early-phase adakitic rocks were generated by the anatexis of Lhasa terrane lower crust owing to crustal shortening and thickening subsequent to the onset of the India–Asia collision and the upwelling of hot asthenosphere beneath the Lhasa terrane caused by the break-off of the Neo-Tethyan oceanic slab. The latest phase of adakitic rocks, however, relates to upwelling asthenosphere following the delamination and/or break-off of the subducting Indian continental slab.

Multistage Metamorphic Evolution of Retrograded Eclogites from the Songshugou Complex, Qinling Orogenic Belt, China

Abstract The Qinling Orogenic Belt is one of the major collisional orogens in eastern Asia and marks the boundary between the North China Craton and South China Craton. The Songshugou complex is the largest basic to ultrabasic body to be found in the North Qinling Belt, and was emplaced as a lens-shaped body at the southern margin of the Qinling Group. A detailed petrological investigation of garnet amphibolite, augen amphibolite and well-foliated amphibolite together with garnet zoning patterns of major and trace elements, inclusions in garnet, and thermodynamic modelling indicate a multistage metamorphic history. Garnets clearly show characteristics of discontinuous growth, as they display optically light-colored snowball-textured cores surrounded by a darker mantle with few inclusions as well as chemically a sudden increase in grossular and decrease in almandine components. A partly resorbed rim is not recognized optically but mineral inclusions and a discontinuous chemical composition of garnet are proof of this third garnet growth stage. Rare earth element distribution patterns of garnet also show clear evidence for discontinuous growth and allow us to identify the reactions responsible for garnet growth. Garnet core compositions as well as amphibole inclusions allow us to constrain a P–T window where this rock equilibrated in a first stage. Calculated pseudosections and the application of the garnet–amphibole thermometer indicate an upper amphibolite- to lower granulite-facies metamorphic episode at 630–740 °C and 0·7–0·9 GPa. The presence of relict omphacite as well as a discontinuously grown garnet mantle with rutile inclusions clearly places the peak metamorphic stage in the eclogite facies. Garnet (XGrs, XAlm, XPrp) and omphacite isopleths (XMg, XNa) constrain this event at 1·7–2·1 GPa and 570–650 °C. Consistent temperatures of 500–650 °C were also determined by clinopyroxene–garnet geothermobarometry for this event. Growth of an outermost rim as well as different stages of garnet breakdown to plagioclase + amphibole coronae and the nearly complete replacement of former omphacite by a variety of symplectites point to an intricate retrograde P–T path. In more strongly retrograded samples plagioclase + amphibole ± quartz pseudomorphs entirely replace former garnet grains. Certain coronae around garnets and symplectites also contain prehnite and pumpellyite, which formed during a late retrograde stage or during a different event at very low P–T conditions (250–350 °C). Based on the detailed petrological study, we favour a multistage metamorphic history of the Songshugou metabasic rocks. The age of the eclogite-facies metamorphic event must be related to the deep subduction of the Songshugou complex during the early Paleozoic, although the age of garnet core growth remains enigmatic. The development of garnet cores indicates an upper amphibolite-facies regional metamorphic overprint succeeded by an eclogite-facies event around 500 Ma and subsequent retrogression seen in replacement of garnet and formation of symplectite. The latest imprint evidenced by prehnite and pumpellyite may be the result of fluid infiltration during the fading orogenic phase or represents a low-temperature overprint by a later process, probably related to the uplift of the North Qinling terrane at around 420 Ma.

Mesoarchean to Paleoproterozoic crustal evolution of the Taihua Complex in the southern North China Craton

The Taihua Complex is exposed mostly in the central-western part along the southern margin of the North China Craton (NCC). In this study, field geological, geochemical, LA-ICP-MS zircon U-Pb age dating and Lu-Hf isotopic analysis are applied to the orthorgneisses, felsic leucosomes, and granites of the Taihua Complex, in order to provide constraints on its crustal evolution from the Mesoarchean to Paleoproterozoic (ca. 2.9–1.8 Ga). The ca. 2.90–2.73 Ga magmatism was an important episode of contemporaneous crustal growth and crustal reworking. Among them, the ca. 2.90 Ga granodioritic enclaves have features similar to those of the Archean sanukitoids, and were generated by the direct partial melting of peridotitic mantle wedge in the subduction processes, representing the early continental nucleus. The ca. 2.84 Ga high Mg# TTGs were derived from the partial melting of subducted oceanic crust after interaction with mantle wedge, whereas the ca. 2.82–2.77 Ga low Mg# TTGs were originated from the partial melting of underplated basaltic crust, pointing to a series of continuous arc magmatic activities. The formation of the ca. 2.75–2.73 Ga crust-derived granites and felsic leucosomes in association with the contemporaneous metamorphism indicate the presence of a crustal anatexis, which probably was the result of an important magmatic underplating event. The magmatism between 2.9 Ga and 2.73 Ga indicates that the increased crust-mantle interaction and crustal differentiation stimulated the crust in the Taihua Complex to become more matural. The ca. 2.57–2.43 Ga magmatism represents another prominent magmatism in the Taihua Complex. The TTG gneisses and dioritic gneisses aged at ca. 2.57–2.50 Ga were derived from partial melting of the hydrated basalts under eclogite to garnet amphibolite facies conditions with interaction between initial melts and mantle wedge, indicating that incessant subduction led to the amalgamation of micro-blocks. The intrusion of granitic rocks, felsic leucosomes, TTG-like gneisses accompanied by metamorphism during ca. 2.50–2.43 Ga indicate that a dominant crustal anatexis event occurred in association with the cratonic stabilization processes. The ca. 2.36–2.24 Ga magmatism was also characterized by reworking of the basement rocks, which are represented by the intrusion of calc-alkaline monzogneisses, A-type granitic gneisses and mafic dykes. This scenario was coeval with the formation of volcanic-sedimentary sequences, indicating the presence of an extension event in the Taihua Complex. The ca. 1.96–1.85 Ga tectono-thermal event represented the last phase of compressional deformation and consequent metamorphism, leading to reworking of the pre-existed basement rocks. This event formed a WNW-ESE trending metamorphic belt that extends over 1000 km along the southern margin of the NCC and was well concordant with the world-wide orogeny associated with the amalgamation of Nuna supercontinent. Subsequently, widespread A-type granites, mafic dykes and volcanic rocks aged of ca. 1.84–1.74 Ga formed, marking the final consolidation of the crystalline basement at the Taihua Complex. Since then, the southern NCC became stable and entered into the stage of development of platform.

Geochemical evidence for reworking of the juvenile crust in the Neoarchean for felsic magmatism in the Yunzhongshan area, the North China Craton

The North China Craton experienced two phases of tectonothermal events at ca. 2.8–2.7 Ga and ca. 2.6–2.5 Ga, respectively. Although 2.6–2.5 Ga granitic gneisses and metavolcano-sedimentary sequences are widespread in the craton, the major stage of crustal growth has been determined to occur at 2.8–2.7 Ga in terms of whole-rock Sm-Nd and zircon Lu-Hf isotope studies. It has been intriguing how the vast 2.8–2.7 Ga juvenile mafic crust would have differentiated into the 2.6–2.5 Ga felsic crust. This issue is addressed in the present study by investigating zircon U-Pb ages and Lu-Hf isotopes as well as whole-rock major-trace elements and Sm-Nd isotopes in Neoarchean felsic igneous rocks from the Yunzhongshan area in the central part of the North China Craton. Zircon U-Pb dating of felsic volcanics, TTG gneisses and high-K granites yields three groups of crystallization ages at 2562–2508 Ma, 2537–2504 Ma and 2518–2508 Ma, respectively, consistent with ages for mafic volcanics in the target area. Zircon Lu-Hf and whole-rock Sm-Nd isotope data indicate that all these rocks were ultimately originated from the ca. 2.8–2.7 Ga juvenile mafic crust. The felsic volcanics are subdivided into low-Si and high-Si suites. The low-Si rocks have higher MgO and FeO contents and contain abundant 2.8–2.7 Ga relict zircons with an age peak of 2.72 Ga, and were produced by partial melting of ~2.72 Ga TTG at ~730 to 750 °C. The high-Si volcanics were derived from partial melting of mafic restites after extraction of the ~2.72 Ga TTG, with significant amounts of garnet and amphibole, but no or rare plagioclase as residual phases, corresponding to a melting pressure at ~15 kbar. The TTG gneisses were produced by partial melting of ~2.8 to 2.7 Ga garnet amphibolites at ~10 to 15 kbar with variable amounts of garnet, amphibole and plagioclase in the residue. The high-K granites were produced by partial melting of ~2.8 to 2.7 Ga medium-high K mafic rocks at pressures of <10 to 12 kbar with a considerable amount of plagioclase in the residue. The TTG rocks and high-Si volcanics require a subduction origin to account for their enrichment in LILE, so that they were mostly likely derived from partial melting of the older arc crust with moderate enrichment in LILE and LREE. By contrast, the ultimate sources of the high-K granites would be the ~2.8 to 2.7 Ga medium-high K basalts, which have lower Zr/Sm ratios than the TTG gneisses and felsic volcanics, and even lower than the bulk continental crust, but consistent with modern island arc basalts. It is possible that the 2.8–2.7 Ga magmatism was subduction-related, with a compositional change of the mafic arc crust from relatively sodic to more potassic at around 2.7 Ga. This may be caused by accretion of the ancient oceanic arc onto the ancient continental margin. It was followed by both growth of the juvenile crust and reworking of the older arc crust in the late Neoarchean. Such series of magmatic processes can account for the voluminous generation of the 2.56–2.50 Ga felsic and mafic igneous rocks in the central part of the North China Craton. Taken together, both growth and reworking of the juvenile crust were realized through accretionary and rifting orogeneses, marking the operation of plate tectonics in the Neoarchean.

Pressure-temperature-time paths from the Funeral Mountains, California, reveal Jurassic retroarc underthrusting during early Sevier orogenesis

Abstract New metamorphic pressure-temperature (P-T) paths and Lu-Hf garnet ages reveal a temporal correlation between Middle to Late Jurassic retroarc underthrusting and arc magmatism in southwestern North America. P-T paths were determined for 12 garnet porphyroblasts from six samples from the Chloride Cliff area of the Funeral Mountains in southeastern California. The composite path shows a pressure increase from 4.2 to 6.5 kbar as temperature increased from 550 to 575 °C, followed by a pressure decrease to 5.1 kbar during a further increase in temperature to 590 °C. Lu-Hf garnet ages from a pelitic schist (167.3 ± 0.7 Ma) and a garnet amphibolite (165.1 ± 9.2 Ma) place these P-T paths in the Middle Jurassic. We interpret the near-isothermal pressure increase portion of the P-T path to have developed during thrust-related burial, similar to lower grade rocks at Indian Pass, 8 km to the southeast, where garnet P-T paths show a pressure increase dated by the Lu-Hf method at 158.2 ± 2.6 Ma. We interpret the pressure decrease portion of the composite P-T path from the Chloride Cliff area to reflect exhumation contemporaneous with cooling in the Indian Pass area documented from muscovite 40Ar/39Ar step-heating ages of 152.6 ± 1.4 and 146 ± 1.1 Ma. The conditions and timing of metamorphism determined for the Indian Pass and Chloride Cliff areas, and isogradic surfaces that cut across stratigraphy, support the interpretation that the strata were dipping moderately NW during metamorphism, parallel to the thrust ramp that buried the rocks. Burial likely resulted from top-SE motion along the Funeral thrust, which was later reactivated as a low-angle normal fault with opposite motion to become the currently exposed Boundary Canyon detachment that was responsible for Miocene and possibly older exhumation. The part of the burial history captured by garnet growth occurred ∼6 m.y. before the 161 Ma peak of high-flux magmatism in the arc. Burial was contemporaneous with metamorphic ages from the western Sierra Nevada metamorphic belt, with the possible timing of accretion of arc terranes in northern California, and with the initiation of Franciscan subduction. Burial ages are also similar in timing with generally E-W crustal shortening in the retroarc that produced the East Sierra thrust system, the Luning-Fencemaker fold and thrust belt, the possible early history of the Central Nevada thrust belt, and the western thrusts of the southern Sevier belt. The timing of tectonic burial documented in this study and of high-flux magmatism in the arc supports the interpretation that the development of a coherent arc-trench system in the Early Jurassic resulted in the underthrusting of melt-fertile material beneath the arc along west- to northwest-dipping faults such as the Funeral thrust in the Jurassic, which penetrated the basement to the west as well as the roots of the magmatic arc, leading to increased magmatism.

The lower crust of the Gangdese magmatic arc, southern Tibet, implication for the growth of continental crust

Magmatic arcs are thought to be the primary sites of modern-day continental crustal growth, and arc crustal sections provide an exceptional opportunity to directly observe the geological processes that occur there, yet few deeply exposed arc sections are available for direct study. The Gangdese magmatic arc, southern Tibet, formed during the Mesozoic subduction of Neo-Tethyan oceanic lithosphere and Cenozoic collision between the Indian and Asian continents, and represent juvenile continental crust. However, the petrological components and compositions of the lower crust of the Gangdese arc remain unknown. Based on detailed geological mapping, we conducted a systemic geochemical, geochronological and zircon Hf isotopic study of well-exposed high-grade metamorphic and migmatitic rocks from the lower crust of the eastern Gangdese arc. The results obtained show that Late Cretaceous garnet amphibolites, dioritic and granitic gneisses, and Paleocene–Eocene garnet amphibolites and granitic gneisses are the main components of the Gangdese lower arc crust. These meta-intrusive rocks witnessed a long period of magmatic, and metamorphic and anatectic processes from the Middle Jurassic to the Late Eocene, and have chemical compositions that range from ultramafic to felsic, with an average SiO2 content of 57.61 wt% and Mg# value of 0.49. These new data indicate firstly that the Gangdese lower arc crust has an overall intermediate composition and typical feature of juvenile crusts, and therefore supports the recent proposition that continental lower crusts are relatively felsic in composition, instead of mafic. We consider that the downward transport of felsic intrusives and associated sedimentary rocks into the deep crustal levels and subsequent partial melting resulted in componential and compositional changes of the Gangdese arc lower crust over time. This is a potential key mechanism in transforming primary lower arc crust to mature continental lower crust for the magmatic arcs with a complete growth history.

Mineralogy and geothermo-barometry of metapelitic schists, amphibolites and garnet amphibolites from Gol-Gohar metamorphic complex, SW Sirjan, Central Iran.

Mineralogy and geothermo-barometry of metapelitic schists, amphibolites and garnet amphibolites from Gol-Gohar metamorphic complex, SW Sirjan, Central Iran.

Zirconium in rutile thermometry of the Himalayan ultrahigh-pressure eclogites and their retrogressed counterparts, Kaghan Valley, Pakistan

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.

Geochemical variations of the Late Mesozoic granitoids in the southern margin of North China Craton: A possible link to the tectonic transformation from compression to extension

The composition of the continental crust of the North China Craton (NCC) is more felsic than that of the average bulk crust, which is regarded to be the result of the delamination of the thickened lower crust during Mesozoic. However, whether the thickened continental crust existed and when the delamination event happened along the southern margin of the NCC are still debated. Here, we report geochronological, geochemical and Sr-Nd-Hf-Pb isotopic evidence that granitoids from the Late Jurassic Wuzhangshan pluton and the Early Cretaceous Huashani complex were derived by partial melting of the lower crust with different thickness. Our new data shows that the two lithofacies of the Wuzhangshan pluton were mainly formed between ca. 157 and 156 Ma, whereas the five lithofacies of the Huashani complex were mainly emplaced between ca. 132 and 125 Ma. The Wuzhangshan pluton and the earlier four lithofacies granitoids of the Huashani complex (ca. 160–125 Ma) both display adakitic geochemical features, which are characterized by as high SiO2 (63.26–72.71 wt%), Al2O3 (13.97–16.89 wt%) and Sr (413–1218 ppm) contents, and low Y (6.30–14.98 ppm) and YbN (1.55–4.43), and high Sr/Y (33−112) and (La/Yb)N (11.53–29.72) ratios. They also have high (87Sr/86Sr)i (0.7066–0.7086), and low εNd(t) (−9.9 to −18.8) and εHf(t) (−11 to −26) values, and two-stage Nd and Hf model ages ranging from 2.4 to 1.7 Ga and 2.7 to 1.7 Ga, respectively. In contrast, the late Early Cretaceous (ca. 125–110 Ma) granitoids have higher SiO2 (71.30–76.78 wt%) and lower Sr (64–333 ppm) contents, and lower Sr/Y (17–29) and (La/Yb)N (13.25–18.36) ratios, and similar εNd(t) (−10 to −16) and relatively higher εHf(t) (−10 to −14) values. These geochemical variations suggest that the ca.160–125 Ma granitoids were most probably produced by partial melting of thickened crust (>45 km) with eclogite, garnet amphibolite or amphibolite residues, whereas that the ca. 125–110 Ma granitoids were formed by partial melting of the thinner crust (<33 km). We thus suggest that the NCC likely underwent a synchronous tectonic transformation at ca. 125 Ma from a compressional setting with thickened crust to an intensive extensional setting with thinner crust at ca. 125 Ma, which demonstrates that the lower crust was most likely delaminated.

Early Cretaceous adakite from the Atlas porphyry Cu-Au deposit in Cebu Island, Central Philippines: Partial melting of subducted oceanic crust

In the Philippine island arc system, there are widely developed Cenozoic arc volcanic rocks, adakites and large Cu-Au deposits. Besides, only two early Cretaceous adakitic intrusions have been found in the entire Philippines so far, i.e., the Lutopan Diorite and the adjacent Kansi diorites in Cebu Island, Central Philippines. The Lutopan Diorite (diorite to quartz diorite in lithology) is the ore-forming intrusion of the giant Atlas porphyry Cu-Au deposit, which is the only Cretaceous porphyry deposit in the Philippines. In this study, geochronology and geochemistry of the Lutopan Diorite in the Carmen pit of Atlas porphyry Cu-Au deposit are investigated in detail, by comparing them with the previously studied Early Cretaceous Kansi diorites and arc volcanic rocks in Cebu Island, to reveal the petrogenesis and magma sources. The Carmen diorites are characterized by moderate SiO2 and MgO contents, coupled with high Na2O and low K2O contents, LREE-enriched and listric REE distribution patterns, low Yb and Y contents, high Sr contents and thereby high Sr/Y ratios, and low (87Sr/86Sr)i (0.7037–0.7038), typical of modern island arc adakites. Zircon U-Pb ages of the Carmen diorites are 109 Ma − 107 Ma in the Early Cretaceous, which is identical with that of the Kansi diorite in the north within error. Both the Carmen and Kansi diorites were formed in intra-oceanic island arc settings. The Carmen diorites show listric REE patterns and low Nb/Ta, high Zr/Sm ratios, suggesting they are formed by partial melting of subducted oceanic crust in amphibolite facies, which is different from the garnet amphibolite- to amphibole eclogite-melting genesis of the Kansi diorites. There is a small amount of subducted sediments in the magma source of Carmen diorite, as further revealed by εNd(t) (+7.1 to +10.6), zircon εHf(t) (+3.3 to +3.9) values, and low Th/U ratios. The Carmen adakitic magma did not undergo significant interaction with mantle peridotite during magma ascent as indicated by the moderate MgO content and Mg# value, and quite low Cr and Ni contents, distinguished from the Kansi diorite (high-Mg adakite) that have high MgO, Mg#, and Cr and Ni contents. We suggest that the nearly contemporary Lutopan Diorite and Kansi diorite are two independent Early Cretaceous adakitic intrusions in Cebu Island, although they are both closely associated with Cu-Au mineralization. The slab-melting genesis and high oxygen fugacities of the Lutopan Diorite are the most important factors to cause Cu enrichment in the magma, which is necessary to form the giant Atlas porphyry Cu-Au deposit.