high-T Metamorphism Research Articles

Geochemical evidence for evolving Proterozoic crustal thickness and orogenic styles in southwestern Laurentia

It has long been challenging for researchers to track the crustal thickness and mode(s) of crustal modification in ancient convergent margins, limiting evaluation of the tectonic styles and processes that modify continental crust during orogenesis. We present trace element igneous geochemical crustal thickness proxies that quantitatively track the crustal thickness evolution of the long-lived Proterozoic active margin in the southwestern U.S.A. We integrate these results with geobarometric data to constrain the mode of crustal modification. The data indicate a complex record of crustal thickness change in space and time and evolving orogenic styles. Geochemical proxies at 1.84–1.72 Ga are consistent with 20–40 km thick magmatic arcs that were locally thickened to ∼50 km during ∼1.75 Ga tectonism. During the Yavapai orogeny, 1.72–1.69 Ga, a ∼200-km-wide belt of 50-60 km thick crust extended from southern California to northern Colorado and was rapidly thinned and exhumed by ∼1.68 Ga. Crustal thickening and thinning during the Yavapai orogeny largely occurred by shortening and exhumation, respectively, in the upper 25 km of the crust. Subsequent 1.68–1.60 Ga tectonism involved crustal growth, local crustal thickening, and low-P, high-T metamorphism, consistent with extensional accretionary orogenesis. The 1.47–1.37 Ga Picuris orogeny was associated with 50–60 km thick crust across much of the Southwest and involved crustal shortening with ∼10 km of magmatic underplating. Advective heat from the emplacement of ferroan granites in the mid-crust likely contributed to elevated geothermal gradients and rheologically weakened the crust. Our results suggest evolving orogenic styles in the Southwest from 1.75–1.69 Ga short-lived crustal thickening associated with terrane accretion to 1.69–1.60 Ga largely extensional accretionary orogenesis, and regional, long-lived crustal thickening at 1.47–1.37 Ga involving extensive basaltic underplating. Contrasting with some recent hypotheses, our data document a complex middle Proterozoic record for the Southwest that was not orogenically quiescent or tectonically stagnant but involved complex mountain building styles.

Drowned in granite - retrieving the tectono-metamorphic history of the Janub metamorphic complex, the northernmost part of the Arabian-Nubian Shield

The Janub Metamorphic Complex (JMC) in southern Jordan provides new correlative data constraining the transition from compressional to extensional tectonics in the northern Arabian-Nubian Shield. This constraint comes from the identification of extensional mylonitic shear zones affecting both the JMC and some intruding granitoids. The JMC comprises metamorphosed andesitic-dacitic-rhyolitic flows and pyroclastics, meta-volcanogenic sediments, and hornfels. Volcanism ceased by 618 ± 5 Ma. The volcanosedimentary sequence formed in an intra-arc basin in a mature island arc and was then buried and regionally metamorphosed at lower greenschist facies conditions in the waning accretionary phase between 618 and 615 Ma. This is based on the age of crosscutting plutons of the Rumman Suite, which triggered local high-T/low-P metamorphism at ∼ 615 Ma. Rumman granitoids and JMC rocks were deformed by narrow gently-dipping mylonitic shear zones, interpreted as extensional, that ceased operating by 605 Ma, the age of crosscutting undeformed dikes and plutons of the Yutum Suite. At 596 Ma, a late granite intruded the JMC and thermally metamorphosed some of the meta-sediments. This granite was deformed in a Najd-related brittle shear zone at ∼ 590 Ma as recorded by 40Ar/39Ar geochronology. The JMC can be correlated with the higher-grade Abu-Barqa Metamorphic Complex (ABMC), despite tectonics relations being obscured by younger intrusive rocks. The main metamorphic phase in the ABMC occurred at a depth of 18 km at ∼ 620–615 Ma, followed by gradual uplift and high-T metamorphism at 615–610 Ma and exhumation to the surface at ∼605 Ma. Extensional shearing in the JMC and exhumation of the ABMC probably developed in a core-complex like setting triggered by orogenic collapse. Ductile shearing related to collapse ceased before 605 Ma. The 590 Ma brittle shear zone is the first dated Najd-related structure from the basement of Jordan. The central setting of our data allows correlation of metamorphic complexes in Sinai and Saudi Arabia and is consistent with the final assembly of Gondwana, initiation of orogenic collapse, and transition to extensional tectonics occurring at ∼ 620–610 Ma in the northern Arabian-Nubian Shield.

Early Cretaceous partial melting recorded by pelitic gneiss from the Nagasaki Metamorphic Complex, western Kyushu, Japan: initiation of Cretaceous high-T metamorphism at eastern margin of Eurasia

ABSTRACT We present Early Cretaceous partial melting recorded by pelitic gneiss from the Nagasaki Metamorphic Complex, western Kyushu, Japan. The existence of the partially melted rock in the complex may imply initiation of long-lived high-T metamorphism in proto-southwest Japan at Cretaceous eastern margin of Eurasia. The gneisses are composed of three garnet-bearing layers and one felsic lens, and they show mylonitic foliation. Application of Zr-in-rutile geothermometer and quartz-in-garnet Raman geobarometer for inclusion rutile and quartz in garnet provided prograde condition of 670–692°C and 1.01–1.26 GPa and peak condition of 755–769°C and 1.17–1.38 GPa. These conditions correspond to those of lower crustal depths between volcanic arc and forearc where thermal advection by wedge mantle corner flow occurs. Presence of the felsic lens whose mineral assemblage can undergo partial melting under the peak condition and presence of a crystallized melt inclusion in garnet suggest that distinct partial melting occurred during the metamorphism. The concordant U–Pb ages of metamorphic zircon rims are ranging from 131.5 ± 4.3 to 120.1 ± 3.9 Ma. The mean square weighted deviation (MSWD) of the metamorphic age was as large as 6.4, implying a significant duration of high-T condition for growth of metamorphic zircon. The obtained ages are within the period of the magmatic hiatus that has previously proposed around southwest Japan and Korean Peninsula (ca. 160–120 Ma) that is due to stop of wedge mantle corner flow by flatting of subducting slab. Our results suggest that high-T metamorphism associated with partial melting at lower crustal depths along Early Cretaceous proto-southwest Japan has already been initiated at the late stage of the hiatus (ca. 130 Ma). This may be due to slab rollback and trench retreat at eastern margin of Eurasia.

Beach placers of southwestern India: An archive of Precambrian supercontinent growth histories

The southwestern coast of India is known for its rich beach placer deposits. So far, there have been no substantial attempts to determine the provenance of these deposits on a regional scale. Here we present for the first time geochronology, trace element chemistry and Hf isotope systematics of zircon and monazite from representative beach placer deposits along the southwestern coast of India to constrain provenance. Detrital zircon grains display a wide variety of ages, with modes in the ranges of 650–450 Ma, 1100–650 Ma, 2300–1600 Ma, 2800–2300 Ma and 3500–2800 Ma. A high proportion of ages fall within the Neoproterozoic–Late Cambrian range (1066–490 Ma). Growth zoning in zircon grains shows magmatic growth of many generations with ages affected by Neoproterozoic Pb loss, along with overgrowths typical of those formed during high-T metamorphism. Grains of metamorphic zircon, along with grains of monazite from the same samples of sediment, have strong modes between 570 and 470 Ma, indicating derivation from sources in the Southern Granulite Terrane affected by a major Late Neoproterozoic to Cambrian tectonothermal event. Mineral geochemistry of zircon grains indicates continental derivation, especially with older zircon from TTG-type sources. The geochemistry of late Neoproterozoic–Cambrian zircon and monazite are consistent with derivation from amphibolite to granulite-facies source rocks. Zircon ages and Hf isotope systematics can be ascribed to provenance from various source terranes exposed along the Western Ghats. This young rift-flank escarpment formed along the coast at a high angle to the distribution of different Precambrian terranes in the Indian Peninsula. Erosion promoted by a tropical climate and a steep coastal geomorphology, in combination with strong southward along-shore ocean currents, led to the concentration and distribution of heavy minerals from multiple sources along southwestern coast of India. The relative intensity of age modes in detrital zircon differs between localities, demonstrating differing inputs from proximal sources (from rivers dissecting steep terrain) versus distal sources from more northerly cratons and terranes brought to deposition sites by along-shore drift.

Miocene-Pleistocene crustal anatexis in the Namche Barwa massif, southern Tibet

位于喜马拉雅东构造结的南迦巴瓦地块经历了复杂的构造变形、强烈的变质和深熔作用，是研究碰撞造山过程中地壳深熔作用的重要对象。完整地厘定新生代晚期岩浆作用期次对于揭示南迦巴瓦地区的构造演化历史和深部过程具有重要意义。南迦巴瓦地块3件淡色花岗岩样品的锆石U-Pb定年结果显示该地块经历了11.30±0.16Ma和2.59±0.04Ma两期地壳深熔作用，可能与南迦巴瓦地块晚新生代快速隆升和剥蚀相关。南迦巴瓦地块保存了大量的~11Ma变质作用和地壳深熔作用记录指示该时间段为构造活动剧烈期。上新世晚期的淡色花岗岩表明，穹窿的隆升和剥蚀所导致的岩浆作用至少持续到了~2.59Ma，代表了南迦巴瓦地区一次年轻的构造岩浆事件。;The Namche Barwa massif, located in the eastern tectonic junction of Himalayan Orogen, has experienced complex tectonic deformation, intensive metamorphism and anatexis which resulted in various types of leucogranites to investigate the nature of crustal anatexis during the tectonic evolution of collision orogenic belts. Knowledge of the timing of such crustal anatexis in rapidly exhumed high grade metamorphic terranes like the Namche Barwa is important to understand the feedback of rapid near-surface processes (erosion and denudation) and those relatively deep processes (high-T metamorphism and partial melting). Zircon U-Pb dating of three leucogranite samples from the Namche Barwa yields that this block has experienced two episodes of young crustal anatexis (11.30±0.16Ma and 2.59±0.04Ma, respectively), which may be related to rapid denudation and uplift of the Namche Barwa in the Late Cenozoic. A number of studies have demonstrated that various rock types in the Namche Barwa have experienced a major episode of metamorphism and crustal anatexis at~11Ma, indicating intense tectonic activities in this period. Occurrence of leucogranite of late Pliocene in age further suggests that the anatectic processes caused by rapid denudation and uplift in this massif lasted at least to~2.59Ma.

Nd isotope re-equilibration during high temperature metamorphism across an orogenic belt: Evidence from monazite and garnet

Monazite is a powerful U-Th-Pb geochronometer commonly used to determine the tempo of orogenic processes. As a light rare earth element (LREE)-enriched mineral, monazite also records key information for understanding the behavior of Sm-Nd isotope systematics during tectonothermal events. However, Sm-Nd isotope studies of monazite have received far less attention than its use as a U-Pb geochronometer. Here we investigate coupled U-Pb ages and Sm-Nd isotopic compositions of monazite from partially melted metasedimentary rocks in the Jiao-Liao-Ji orogenic belt (JLJB), North China craton, to provide insight into Sm-Nd isotope behavior during high-temperature metamorphism and crustal melting. We utilize several complementary dating systems in the same sample, including garnet Lu-Hf and Sm-Nd, zircon U-Pb, and monazite U-Pb geochronology, to gain precise time constraints on the P-T evolution of the JLJB. Garnet Lu-Hf isochron dates of 1.97–1.94 Ga are interpreted to represent prograde garnet growth, consistent with the prograde metamorphic zircon dates of 1.96–1.90 Ga; garnet Sm-Nd dates agree with retrograde metamorphic zircon dates, and all monazite U-Pb dates define a relatively narrow age range of 1.86–1.83 Ga, interpreted to represent retrograde cooling. The lack of older monazite grains matching zircon or garnet dates suggests the dissolution of pre-existing monazites into partial melt. Monazite in all samples, irrespective of occurring as garnet inclusions or in the matrix, yields homogenous Nd isotope compositions (εNd ~ −5) at ~1.85 Ga, which are consistent with the garnet and bulk-rock values. Thus, Nd isotope re-equilibration between bulk-rock and minerals occurs synchronously across the orogenic belt during high-T metamorphism (~750 °C) where melting was involved, with no evidence of a pre-metamorphic isotope signature. This suggests mobility in the Sm-Nd isotope system during high-grade metamorphism and melting, where the Nd isotope signature of monazite likely represents the most recent tectonothermal event.

Age of metamorphism and deformation in the Montagne Noire dome (French Massif Central): Tapping into the memory of fine-grained gneisses using monazite U-Th-Pb geochronology

Recent work has revealed that the Montagne Noire dome, located in the foreland of the Variscan belt (French Massif Central), contains a record of nearly coeval, late Variscan eclogitization and migmatization. Given these new results, it is important to understand the chronology of events that produced high-grade metamorphism and exhumation. Using U-Th-Pb dating of monazite, this study confirms ages of ~315–300 Ma for high-T metamorphism in the augen gneiss that makes up a large fraction of the Montagne Noire dome, and documents for the first time ~295 Ma monazite ages in compositionally varied fine-grained gneisses that form folded continuous layers within the core of the dome. The fine-grained gneiss layers are intensely sheared and are interpreted to have localized late, high-T deformation in the core of the dome. These sheared fine-grained gneisses form a network of shear zones that were kinematically linked to the extensional and strike-slip deformation zones that exhumed the Montagne Noire dome in a pull-apart (s.l.) domain. Continued deformation-recrystallization and fluid flow within these shear zones likely drove rejuvenation of monazite for ~5 million years after much of the melt had crystallized in the Montagne Noire dome.

The Mangaroon Orogeny: Synchronous c. 1.7 Ga magmatism and low-P, high-T metamorphism in the West Australian Craton

The Capricorn Orogen records nearly one billion years of intraplate orogenesis within the West Australian Craton, although the processes responsible for this protracted, punctuated reworking remain unclear. Of the major tectonic events that affected the region, the 1680–1620 Ma Mangaroon Orogeny is one of the least-well understood, mainly due to a lack of direct ages for metamorphism, an absence of pressure (P)–temperature (T) constraints, and uncertainty regarding the duration of granitic magmatism that is spatially and, possibly, temporally associated with deformation. In this study we define the P–T–time (t) conditions associated with the Mangaroon Orogeny based on in situ SHRIMP U–Pb monazite and xenotime geochronology and calculated P–T pseudosections. Data from a pelitic migmatite constrain the timing of low–P, high–T metamorphism to 1691 ± 7 Ma at conditions of 665–755 °C and 2.7–4.3 kbar (~175–240 °C/kbar). Data from a garnet-bearing sillimanite–biotite pelitic gneiss suggests higher pressure during the clockwise prograde history at one locality. Furthermore, the onset of the metamorphism coincides with the oldest granites in the region (1695 ± 9 Ma), which constrain the onset of the Mangaroon Orogeny. Our results also show that deposition and burial of the precursor sediments occurred, at most, c. 70 million years before the onset of partial melting at c. 1695 Ma. Therefore, there was no long incubation period before the onset of orogenesis. We conclude that, unlike many of the Proterozoic orogenic events in Australia, the c. 1.7 Ga low-P, high-T metamorphism recorded in the upper crust in the Capricorn Orogen cannot be explained by a thermal lid model, but rather was synchronous with granitic magmatism.

Petrology and 40Ar-39Ar dating of paragneisses from the Devrekani Massif (Central Pontides, Northern Turkey): Implications for the Jurassic high-T metamorphism in an extensional tectonic environment

The Devrekani Massif in the northern part of the Central Pontides (Northern Turkey) provides important clues to the regional tectonics and geodynamic processes associated with Jurassic high grade metamorphic conditions. This study reports new paragenetic assemblages, mineral compositions, whole-rock geochemistry and 40Ar-39Ar geochronological data from the paragneisses in the massif, and, discusses the P–T conditions and geodynamic implications of the Jurassic metamorphism during continental extension in the Central Pontides. Upper amphibolite to lower granulite facies paragneisses form one of the main lithological units in the massif. Within these, there are five different mineral parageneses with diagnostic mineral assemblages of: quartz, K-feldspar (An0-1Ab4-26Or73-96), plagioclase (An18-35), biotite [XPhl: 0.28–0.57; Mg/(Mg + Fe2+): 0.33–0.61], sillimanite, cordierite [Mg/(Mg + Fe2+): 0.48–0.71] and garnet (Alm43-80Grs0-18Prp5-23And0-4Sps10-33) with minor hercynite. Based on Na-in-Crd thermometry and GASP barometry results, the peak metamorphic conditions are 775 ± 25 °C and 6 ± 1 kbar in the massif. The field relations, petrography and bulk chemical data suggest that the paragneisses, derived from shale-wackestone and pelitic sedimentary protoliths, are typical rock lithologies of an active continental margin. They display enrichments in LILE over HFSE, coupled with negative Nb and Ti anomalies, which are geochemical signatures of subduction-related sources. It is likely that the peak metamorphism took place during the Middle–Upper Jurassic period (ca. 174–156 Ma), suggesting that the metamorphic rocks cooled to 300–350 °C at ca. 156 Ma. The mineral assemblages reveal that the prograde history passed from sillimanite zone conditions up to the cordierite-garnet-K-feldspar zone. The petrological and geochronological data indicate that the protoliths are related to multiple sources such as volcano-sedimentary successions. We conclude that the Devrekani Massif represents the products of pre-Jurassic sedimentation, and Permo-Carboniferous continental arc magmatism, overprinted by Jurassic metamorphism.

Heterogeneity of Mantle Peridotites from the Polar Urals (Russia): Evidence from New LA-ICP-MS Data

To discuss the nature of the compositional heterogeneity of the peridotite massifs of the Polar Urals (Russia), the geochemical study by LA-ICP-MS of pyroxenes and amphiboles from these mantle formations was performed. The trace element compositions in clinopyroxenes indicate the existence of the mantle protolith of two types. The first protolith type, represented by lherzolites and diopside harzburgites, was originated from the partial melting (5%-10%) under the spinel facies conditions, while the second one, represented by diopside harzburgites, was formed under the polybaric partial melting (17%-19%) under garnet and spinel facies conditions. Subsequently, the mantle peridotite protolith was subject to fluidinduced partial melting in the suprasubduction setting that was resulted in the formation of harzburgites. Being affected by penetrating melts and fluids peridotites experienced the refertilization (LREE enrichment of clinopyroxenes) and high-temperature hydratation with subsequent development of pargasite and Mg amphibole. The high-T fluid-induced metamorphism at the subduction zone was accompanied by the formation of metaperidotites with clinochlore and REE-depleted tremolite.

Gneiss-forming events in the Saglek Block, Labrador; a reappraisal of the Uivak gneiss

The Archean gneiss complex of the Saglek Block of Labrador is a part of the North Atlantic Craton, and is correlated with southern West Greenland, both being metamorphosed during a ca. 2.7 Ga event. The main component of the complex is the Eoarchean Uivak orthogneiss, which includes lenses of the Nulliak supracrustal assemblage. Both lithologies are cut by the mafic Saglek metadykes. The Uivak gneisses have been divided into Uivak I grey gneiss and Uivak II augen gneiss. The former underwent ca. 3.6 Ga high-T metamorphism prior to the intrusion of the latter. However, the exact age, nature, and extent of Uivak II gneiss are poorly understood. We present geochemical and geochronological results for both these orthogneisses to help refine the various hypotheses that have been proposed concerning the nature of their protoliths. Magmatic ages of 3746 ± 5 and 3717 ± 6 Ma are consistent with previous estimates for the age of Uivak I gneiss. Uivak II augen gneiss from Maidmonts Island, where there is a clear intrusive relationship between the Uivak II and Uivak I gneissic protoliths, has an age of 3325 ± 3 Ma. This is similar to an homogeneous grey gneiss from St. John’s Harbour, with an age of 3318 ± 5 Ma. Grey gneiss from Big Island is distinctively younger (3219 ± 7 Ma), and equivalent to the ca. 3.24 Ga Lister gneiss. Our study shows that granitic gneisses classified as Uivak II were emplaced 200–300 million years after ca. 3.6 Ga metamorphism and deformation of the Uivak I gneiss. The igneous protolith of Uivak II gneiss pre-dates the Lister gneiss by about 100 Ma. The Uivak I and Lister gneisses are geochemically similar, and are both Tonalite–Trondhjemite–Granodiorite (TTG) gneisses, whereas the Uivak II gneiss is a granitoid partially derived from pre-existing crust. We propose abandoning the term ‘Uivak II gneiss’, and renaming ca. 3.3 Ga granitoids, after the type locality, as Maidmonts gneiss. This restricts the term ‘Uivak gneiss’ to Eoarchean TTG gneisses and removes the necessity for subdividing them into Uivak I and II.

SHRIMP U-Pb Geochronology of the Socorro Batholith and implications for the Neoproterozoic evolution in SE Brazil

The Socorro Batholith is one of the most expressive granite manifestations associated with the Neoproterozoic evolution in SE Brazil, occupying large areas (~1,200 km 2 ) in the southern portion of the Socorro-Guaxupe Nappe. A U-Pb zircon SHRIMP dating program was developed to determine the ages of the main components of this batholith, identified in previous detailed mapping projects. High-K calc-alkaline (HKCA) porphyritic biotite-hornblende granites with relatively low (60–67 wt%) SiO 2 are the most voluminous component of this and other large “syn-tectonic” batholiths in the SGN (Agua Limpa and Pinhal-Ipuiuna) and neighboring domains located south of it in the Apiai and Sao Roque Domains of the Ribeira Fold Belt. Two samples collected in widely separated localities at the northern and southern part of the Socorro batholith yield similar ages of magmatic crystallization, respectively 610.1 ± 7.0 and 608.3 ± 6.6 Ma. A more fractioned (> 72 wt% SiO 2 ) granite reported in the literature as related to a younger event (“Socorro II magmatism”, as opposed to the previous “Socorro I” HKCA granites) yield a precise age that is clearly older (624.4 ± 3.6 Ma), and contemporary to anatectic granites and migmatites that were produced during a prolonged period of high-grade metamorphism (635–605 Ma) that affected the SGN. Our data thus indicates that at least part of the HKCA magmatism that constitutes the Socorro batholith post-dates the high-P metamorphism associated to continental collision, and may have been a source of heat and volatiles to the high-T metamorphism responsible for partial melting of the upper portions of the crustal section represented by the SGN. Two charnockitic rocks that show transitional contacts with granites of the Socorro batholith were also dated. The Socorro Charnockite is aged 641.6 ± 4.1 Ma, which overlaps those of regional orthogneisses (in part also of charnockitic character) considered as associated with a pre-collisional tectonics (subduction-related?). However, it is reported to transition to granites that are very similar to the HKCA granites of the Socorro batholith, which are yet undated. The Atibaia Charnockite has distinct geochemical affinity (lower mg# and Sr content; higher Zr), a younger age (633.3 ± 6.2 Ma), and may signal a different tectonic setting at the end of the period of plate consumption as yet poorly characterized.

Implications of the distribution, age and origins of the granites of the Mesoproterozoic Spektakel Suite for the timing of the Namaqua Orogeny in the Bushmanland Subprovince of the Namaqua-Natal Metamorphic Province, South Africa

The Spektakel Suite comprises voluminous late- to post-tectonic granitoids that intruded the pre-tectonic gneisses of the Bushmanland Subprovince (BSP) of the Mesoproterozoic Namaqua-Natal Metamorphic Province (NNMP). Based on new U-Pb zircon geochronology and geochemical data, we redefine the suite as consisting of 22 plutons of porphyritic monzogranite, granodiorite and charnockite that intruded the granulitic core of the BSP between 1097 and 1033 Ma. The new data show that some granites previously incorporated into the Spektakel Suite should be excluded: a) garnetiferous leucogranites (Concordia, Hangsfontein and Ibequas Granites) are excluded on petrological grounds and genetic considerations and b) the porphyritic Nuwefontein Granite (∼783 Ma) is younger and is now placed within the Richtersveld Suite. The Spektakel Suite has geochemical and isotopic compositions suggesting that it was derived from recycled Palaeoproterozoic crust with Mesoproterozoic mantle input. The main Namaqua Orogeny (D2) in the BSP comprises at least two phases of compressional tectonics at ∼1200 Ma and ∼1110 Ma separated by a period of extension. The new ages for the Spektakel Suite tightly constrain the end of D2 to between 1109 Ma, the age of the youngest pre-tectonic gneiss and 1097 Ma, the age of the oldest Spektakel Suite granite. Peak low-P, high-T metamorphism (∼1050–1020 Ma) occurred well after D2 and its close spatial and temporal relationship to the Spektakel Suite suggests this prolonged period of granite magmatism was responsible for the transfer of the heat into the BSP. Several other late- to post-tectonic megacrystic granitic suites intruded into other tectonic domains across the NNMP coevally with the Spektakel Suite, indicating widespread granite magmatism at this time, probably due to mafic under- and intra-plating in a back-arc setting as previously suggested.

Hot lherzolite exhumation, UHT migmatite formation, and acid volcanism driven by Miocene rollback of the Banda Arc, eastern Indonesia

The northern Banda Arc, eastern Indonesia, exposes upper mantle/lower crustal complexes comprising lherzolites and granulite facies migmatites of the ‘Kobipoto Complex’. Residual garnet–sillimanite granulites, which contain spinel+quartz inclusions within garnet, experienced ultrahigh-temperature (UHT; >900°C) conditions at 16Ma due to heat supplied by lherzolites exhumed during slab rollback in the Banda Arc. Here, we present U–Pb zircon ages and new whole-rock geochemical analyses that document a protracted history of high-T metamorphism, melting, and acid magmatism of a common sedimentary protolith. Detrital zircons from the Kobipoto Complex migmatites, with ages between 3.4Ga and 216Ma, show that their protolith was derived from both West Papua and the Archean of Western Australia, and that metamorphism of these rocks on Seram could not have occurred until the Late Triassic. Zircons within the granulites then experienced three subsequent episodes of growth – at 215–173Ma, 25–20Ma, and at c. 16Ma. The population of zircon rims with ages between 215 and 173Ma document significant metamorphic (±partial melting) events that we attribute to subduction beneath the Bird's Head peninsula and Sula Spur, which occurred until the Banda and Argo continental blocks were rifted from the NW Australian margin of Gondwana in the Late Jurassic (from c. 160Ma). Late Oligocene-Early Miocene collision between Australia (the Sula Spur) and SE Asia (northern Sulawesi) was then recorded by crystallisation of several 25–20Ma zircon rims. Thereafter, a large population of c. 16Ma zircon rims grew during subsequent and extensive Middle Miocene metamorphism and melting of the Kobipoto complex rocks beneath Seram under high- to ultrahigh-temperature (HT–UHT) conditions. Lherzolites located adjacent to the granulite-facies migmatites in central Seram equilibrated at 1280–1300°C upon their exhumation to 1GPa (~37km) depth, whereupon they supplied sufficient heat to have metamorphosed adjacent Kobipoto Complex migmatites under UHT conditions at 16Ma. Calculations suggesting slight (~10vol%) mantle melting are consistent with observations of minor gabbroic intrusions and scarce harzburgites. Subsequent extension during continued slab rollback exhumed both the lherzolites and adjacent granulite-facies migmatites beneath extensional detachment faults in western Seram at 6.0–5.5Ma, and on Ambon at 3.5Ma, as recorded by subsequent zircon growth and 40Ar/39Ar ages in these regions. Ambonites, cordierite- and garnet-bearing dacites sourced predominantly from melts generated in the Kobipoto Complex migmatites, were later erupted on Ambon from 3.0 to 1.9Ma.

A revisit to the Higo terrane, Kyushu, Japan: The eastern extension of the North China–South China collision zone

The Higo and Manotani high-T metamorphic rocks belong to the Higo terrane, central Kyushu, Japan. This terrane has been suggested as the eastern extension of the North China–South China collision zone based mainly on the similar petrological and geochronological data to the north Dabie high-grade rocks. In this study, detrital zircon U–Pb dating for the Higo metamorphic rocks yielded a maximum depositional age of 197Ma with a prominent component at 193–284Ma (62–95%) and a subsidiary peak at 1847–1875Ma (19–28%); whereas for the Manotani metamorphic rocks, the maximum depositional age is 263Ma with a prominent component at 263–304Ma (98%). These results suggest that the protoliths of the Higo and Manotani metamorphic rocks might be equivalent to those of the early–middle Jurassic Chizu and late Triassic Suo high-P metamorphic rocks, southwest Japan, respectively. The absence of middle Neoproterozoic detrital zircon (c. 700–825Ma) is incompatible with a proposition that the proto-Japan might have been located close to the South China craton, where Permian–Jurassic sedimentary rocks contain a significant amount of middle Neoproterozoic detrital zircon. The zircon provenance of the Higo terrane, as well as of the Chizu and Suo belts, could therefore be the North China craton, suggesting that the Suo metamorphic rocks, including the Manotani metamorphic rocks, have formed at the subduction plate boundary in the east, which changed into the contemporaneous North China–South China collision plate boundary in the west, likely passing through the Korean Peninsula. The transformation of the Chizu and Suo metamorphic rocks to the Higo and Manotani metamorphic rocks can be accounted for by the oceanward advance of the volcanic arc during trench retreating. The Cretaceous high-T metamorphism and related igneous intrusions of the Higo terrane would be regarded as the initial phase of the Ryoke event.

Dredge samples from the Chukchi Borderland: Implications for paleogeographic reconstruction and tectonic evolution of the Amerasia Basin of the Arctic

The Chukchi Borderland is a large bathymetric high that extends from the Alaskan Chukchi Shelf into the Amerasia Basin of the Arctic Ocean. Widely interpreted to be underlain by continental crust, the Chukchi Borderland has played a pivotal role in plate reconstructions of the Arctic, despite the fact that its geologic nature, origin and evolution are largely unknown. We present new lithologic descriptions, along with U-Pb, 40Ar/39Ar, and apatite fission track analyses, for rocks dredged from the Chukchi Borderland. These new results provide constraints on the Chukchi Borderland9s crustal architecture, Paleozoic evolution, and pre-Cretaceous paleogeographic position prior to the formation of the Amerasia Basin. Based on the location and nature of dredged rocks, the Chukchi Borderland comprises at least two distinct terranes, juxtaposed by the proposed Chukchi Borderland fault zone. Early Paleozoic units recovered from Northwind Ridge in the eastern Chukchi Borderland consist of low-grade metasedimentary rocks with a Laurentian detrital zircon signature that is statistically indistinguishable from that of Cambrian age units of the Franklinian passive margin exposed on Ellesmere Island in the Canadian Arctic Archipelago. In contrast, high-grade gneisses and amphibolites dredged from the Chukchi Plateau in the central Chukchi Borderland yield U-Pb and 40Ar/39Ar ages that suggest an affinity with rock units of the Pearya terrane of Arctic Canada. U-Pb and 40Ar/39Ar ages reveal a Paleozoic tectonic history beginning in Early Cambrian to Early Ordovician time with subduction/arc-related high-T metamorphism. Intrusive relationships indicate that Late Ordovician to Silurian orthogneisses intruded the high-T units within an arc setting. Devonian exhumation of these high-grade units is recorded by U-Pb dating of sphene and 40Ar/39Ar thermochronology of micas and potassium feldspar. The exhumation of the high-grade rocks was coeval with the deposition of coarse-grained sediments bearing detrital zircon compositions similar to Silurian foreland basin sediments deposited in Pearya, as well as with greenschist facies metamorphism of metasedimentary rocks dredged from Northwind Ridge. The new data suggest paleogeographic restoration of the Chukchi Borderland to a position adjacent to Ellesmere and Axel Heiberg islands prior to Amerasia Basin rifting. This pre-rift position supports and refines the widely cited rotational models for opening of the Canada Basin.

New evidence for Palaeoproterozoic high grade metamorphism in the Trivandrum Block, Southern India

In-situ zircon U–Pb isotopic and monazite Th–U–Pb chemical ages from leucosome in migmatitic paragneiss at Kanjampara in the Trivandrum Block, India, demonstrate the occurrence of high-T metamorphism and anatexis at 1.92Ga. This Palaeoproterozoic tectonothermal event was strongly overprinted by the main Neoproterozoic-Cambrian granulite facies tectonism which itself involved significant partial melting and formation of garnet-bearing assemblages at 0.65GPa and >820–860°C. Monazite grains, which enclose zircon grains and locally preserve Palaeoproterozoic chemical age domains, were extensively reset and recrystallized at 565±6Ma. These monazite grains were further modified to form high-Th cuspate rims at 517±15Ma, equivalent to the lower intercept age for extensive Pb-loss from the highly discordant 1.92Ga zircon, 528±18Ma. These results confirm that at least some of the metasedimentary paragneisses in the Trivandrum Block are polymetamorphic, initially metamorphosed in the Palaeoproterozoic in an event older than the 1.89–1.85Ga granitic orthogneisses recognised from the area. The nature of the relationship between the paragneisses and older (2.0–2.1Ga) granitic and charnockitic orthogneisses of the Trivandrum and Nagercoil Blocks, in particular whether they share the same Palaeoproterozoic history or were interleaved after 1.92Ga, requires further investigation focussed on the presence or absence of the isotopic imprint of this event in both rock suites throughout the area.The growth or recrystallisation of monazite at ca. 565Ma and its further modification at ca. 520Ma indicates that the high-T metamorphism in the Pan-African was long-lived, with a duration of at least 45Myr. This adds further weight to recent proposals, based on zircon-monazite isotopic studies from other localities in the Trivandrum Block, that the region formed the mid- to deep-crust of a long-lived collisional hot orogenic belt that welded Gondwana from ca. 580–510Ma.

Metamorphism and partial melting of ordinary chondrites: Calculated phase equilibria

Constraining the metamorphic pressures (P) and temperatures (T) recorded by meteorites is key to understanding the size and thermal history of their asteroid parent bodies. New thermodynamic models calibrated to very low P for minerals and melt in terrestrial mantle peridotite permit quantitative investigation of high-T metamorphism in ordinary chondrites using phase equilibria modelling. Isochemical P–T phase diagrams based on the average composition of H, L and LL chondrite falls and contoured for the composition and abundance of olivine, ortho- and clinopyroxene, plagioclase and chromite provide a good match with values measured in so-called equilibrated (petrologic type 4–6) samples. Some compositional variables, in particular Al in orthopyroxene and Na in clinopyroxene, exhibit a strong pressure dependence when considered over a range of several kilobars, providing a means of recognising meteorites derived from the cores of asteroids with radii of several hundred kilometres, if such bodies existed at that time. At the low pressures (<1 kbar) that typify thermal metamorphism, several compositional variables are good thermometers. Although those based on Fe–Mg exchange are likely to have been reset during slow cooling, those based on coupled substitution, in particular Ca and Al in orthopyroxene and Na in clinopyroxene, are less susceptible to retrograde diffusion and are potentially more faithful recorders of peak conditions. The intersection of isopleths of these variables may allow pressures to be quantified, even at low P, permitting constraints on the minimum size of parent asteroid bodies. The phase diagrams predict the onset of partial melting at 1050–1100 °C by incongruent reactions consuming plagioclase, clinopyroxene and orthopyroxene, whose compositions change abruptly as melting proceeds. These predictions match natural observations well and support the view that type 7 chondrites represent a suprasolidus continuation of the established petrologic types at the extremes of thermal metamorphism. The results suggest phase equilibria modelling has potential as a powerful quantitative tool in investigating, for example, progressive oxidation during metamorphism, the degree of melting and melt loss or accumulation required to produce the spectrum of differentiated meteorites, and whether the onion shell or rubble pile model best explains the metamorphic evolution of asteroid parent bodies in the early solar system.

Early Permian mantle–crust interaction in the south-central Altaids: High-temperature metamorphism, crustal partial melting, and mantle-derived magmatism

Early Permian mafic magmatism, the partial melting of the crust, and high-temperature metamorphism in the Chinese Altai, south-central Altaids, provide an excellent case study of exchange of energy and mass between the mantle and crust. Field and petrographic observations, together with microprobe mineral analyses, have, for the first time, allowed us to identify scapolite-bearing calc-silicate granulites along the southern margin of the Chinese Altai. The conditions of metamorphism were 680–800°C and 6–7 Kbar, based on mineral phase relationship and compositions, as well as the results of previous studies. New LA-ICP-MS zircon U-Pb geochronology demonstrates that the high-T metamorphism in the Chinese Altai was accompanied by the emplacement of leucogranites at approximately 295Ma. SHRIMP II dating results, combined with previously published data, reveal a mafic magmatic event at around 275Ma in a large region of northern Xinjiang, NW China. This mafic magma was derived from N-MORB-like depleted mantle, as deduced from our new bulk geochemical and zircon Hf isotopic data. Synthesizing available geochemical (including isotopic) and geochronological data, we propose a two-stage model of mantle–crust interaction to explain the early Permian geology of northern Xinjiang. The early stage of interaction involved high-temperature metamorphism and the coeval partial melting of crustal rocks, indicating solely a heat exchange between the mantle and crust; in contrast, the later stage involved the exchange of both mass and energy between the mantle and crust.

DEFORMATION AND AGE OF THE STINKBANK PLUTON AND IMPLICATIONS FOR THE CORRELATION OF TECTONOMETAMORPHIC EPISODES IN THE PAN-AFRICAN DAMARA BELT

U-Pb zircon and monazite ages from the oldest and youngest granite phases of the composite Stinkbank pluton in the south Central Zone of the Pan-African Damara belt in Namibia indicate a timing of emplacement of the composite pluton between 547.5 ± 4.7 Ma and 529.4 ± 4 Ma. This documents the assembly of the sheet-like pluton over a period of 17 ± 7 Ma (2σ ) in the lower Cambrian. Intrusive relationships, the folding of all granite phases and the pervasive development of magmatic and solid-state foliations indicate the syn-tectonic emplacement and deformation of the pluton during northwest-southeast shortening (D3) over a protracted time between >550 Ma and <530 Ma. On a regional scale, D3 shortening strains related to the high-angle convergence between the Kalahari and Congo Cratons can be recorded over a period of >50 Ma in the south Central Zone, between >560 Ma and at least 500 Ma. We suggest that previously identified deformation episodes do not represent distinct deformational phases related to changes in regional stress fields or plate kinematics. Instead, different kinematics and the seemingly different timing of deformational episodes are the result of variations of the strength of the Central Zone during high-angle convergence. Mid- and lower-crustal levels are decoupled from the upper crust, forming a ductile, partially molten orogenic infrastructure beneath a rheologically stronger, upper-crustal superstructure. Variations in crustal rheologies are brought about by regional high-T metamorphism and, in particular, partial melting of mid- and lower crustal levels. The presence of melts and advective heat transport through granite plutons has resulted in a similar weakening of even upper crustal levels, although spatially restricted to the intrusions and immediate wall rocks. The orogenic superstructure-infrastructure also implies that the correlation of deformational events through seemingly syn-, late- and post-tectonic granite intrusions from different parts and levels of the orogen may lead to erroneous interpretations of the actual timing of events.