Timing Of Ultrahigh-pressure Metamorphism Research Articles

UHP metamorphism recorded by coesite-bearing metapelite in the East Kunlun Orogen (NW China)

AbstractThe East Kunlun Orogen (EKO) is the NW part of the Central China Orogenic Belt, which records the evolutionary history of the Proto- and Palaeo-Tethys Oceans from the Cambrian to the Triassic. An Early Palaeozoic eclogite belt has been recognized in recent years, which extends discontinuously for ∼500 km as three eclogite-bearing terranes. In this study, we report an integrated study of zircon grains from mica-schists accompanying the eclogites, in terms of mineral inclusions, U–Pb age systematics andP–Tconditions. The presence of coesite is identified, as inclusions within the metamorphic domain of zircons, which provides unambiguous evidence for subducted terrigenous clastic rocks of the Proto-Tethys Ocean exhumed from coesite-forming depths. U–Pb dating of the metamorphic zircons yields a concordia age of 426.5 ± 0.88 Ma, which is likely to be the time of ultrahigh-pressure metamorphism in the Kehete terrane.P–Tcalculations suggest that metapelite may have experienced a clockwiseP–Tpath with peakP/Tconditions of 685 ± 41 °C and >28 kbar, and equilibrated at 482–566 °C and 5.6–8.9 kbar during subsequent exhumation. The high-pressure – ultrahigh-pressure (HP-UHP) metamorphic belt within the EKO may have formed by collision between the Qaidam Block and the South Kunlun Block, as a consequence of the closure of the Proto-Tethys Ocean.

Timing of subduction initiation, arc formation, ophiolite obduction and India–Asia collision in the Himalaya

Abstract Reconstruction of the Western Himalaya requires three subduction systems operating beneath the Spong arc, Dras–Kohistan arc and the Asian continent during the Late Cretaceous–Paleocene. The timing of the closure of the Neo-Tethys Ocean along the Indus Suture Zone (ISZ) in Ladakh and south Tibet has been proposed to be as old as c. 65 Ma and as young as c. 37 Ma. The definition of the India–Asia collision can span >15 myr from the first touching of Indian continental crust with Asian crust to the final marine sedimentation between the two plates. There is good geological evidence for a Late Cretaceous–Early Paleocene phase of folding, thrusting and crustal thickening of Indian Plate shelf carbonates associated with obduction of ophiolites. There is no geological evidence of any oceanic ‘Greater Indian Basin’ separating the northern Tethyan and Greater Himalaya from India. There is clear evidence to support final ending of marine sedimentation along the ISZ at 50 Ma (planktonic foraminifera zone P7–P8). There is no evidence for diachroneity of collision along the Pakistan–Ladakh–South Tibet Himalaya. The timing of ultrahigh-pressure metamorphism cannot be used to constrain India–Asia collision, and the timing of high-grade kyanite- and sillimanite-grade metamorphism along the Greater Himalaya can only give a minimum age of collision.

Timing of metamorphism and exhumation in the Nordøyane ultra-high-pressure domain, Western Gneiss Region, Norway: New constraints from complementary CA-ID-TIMS and LA-MC-ICP-MS geochronology

We present the results of a combined CA-ID-TIMS and LA-MC-ICP-MS U–Pb geochronology study of zircon and associated rutile and titanite from the Nordøyane ultra-high-pressure (UHP) domain in the Western Gneiss Region (WGR) of Norway. The dated samples include 4 eclogite bodies, 2 host-rock migmatites, and 2 cross-cutting pegmatites and leucosomes, all from the island of Harøya. Zircon from a coesite eclogite yielded an age of ca. 413 Ma, interpreted as the time of UHP metamorphism in this sample. Zircon data from the other eclogite bodies yielded metamorphic ages of ca. 413 Ma, 407 Ma, and 406 Ma; zircon trace-element data associated with 413 Ma and 407 Ma ages are consistent with eclogite-facies crystallization. In all of the eclogites, U–Pb dates from zircon cores, interpreted as the times of protolith crystallization, range from ca. 1680–1586 Ma, consistent with Gothian ages from orthogneisses in Nordøyane and elsewhere in the WGR. A zircon core age of ca. 943 Ma from one sample agrees with Sveconorwegian ages of felsic gneisses and pegmatites in the western part of the area. Migmatites hosting the eclogite bodies yielded zircon core ages of ca. 1657–1591 Ma and rim ages of ca. 395–392 Ma, interpreted as the times of Gothian protolith formation and Scandian partial melt crystallization, respectively. Pegmatite in an eclogite boudin neck yielded a crystallization age of ca. 388 Ma, interpreted as the time of melt crystallization. Rutile and titanite from 3 samples (an eclogite and two migmatites) yielded concordant ID-TIMS ages of 378–376 Ma. The results are similar to existing U–Pb data from other Nordøyane eclogites (415–405 Ma). In combination with previous pressure-temperature data from the coesite eclogite, these ages indicate that peak metamorphic conditions of ~3 GPa/760 °C were reached ca. 413 Ma, followed by decompression to ~1 GPa/810 °C by ca. 397 Ma and cooling below ca. 600 °C by ca. 375 Ma. The results are compatible with protracted UHP metamorphism followed by relatively slow exhumation. The question of whether partial melting began at UHP conditions is not resolved by this study.

Triassic to Early Jurassic (c. 200 Ma) UHP metamorphism in the Central Rhodopes: evidence from U–Pb–Th dating of monazite in diamond‐bearing gneiss from Chepelare (Bulgaria)

AbstractEvidence for ultrahigh‐pressure metamorphism (UHPM) in the Rhodope metamorphic complex comes from occurrence of diamond in pelitic gneisses, variably overprinted by granulite facies metamorphism, known from several areas of the Rhodopes. However, tectonic setting and timing of UHPM are not interpreted unanimously. Linking age to a metamorphic stage is a prerequisite for reconstruction of these processes. Here, we use monazite in diamond‐bearing gneiss from Chepelare (Bulgaria) to date the diamond‐forming UHPM event in the Central Rhodopes. The diamond‐bearing gneiss comes from a strongly deformed, lithologically heterogeneous zone (Chepelare Mélange) sandwiched between two migmatized orthogneiss units, known as Arda‐I and Arda‐II. Diamond, identified by Raman micro‐spectroscopy, shows the characteristic band mostly centred between 1332 and 1330 cm−1. The microdiamond occurs as single grains or polyphase diamond + carbonate inclusions, rarely with CO2. Thermodynamic modelling shows that garnet was stable at UHP conditions of 3.5–4.6 GPa and 700–800 °C, in the stability field of diamond, and was re‐equilibrated at granulite facies/partial melting conditions of 0.8–1.2 GPa and 750–800 °C. The texture of monazite shows older central parts and extensive younger domains which formed due to metasomatic replacement in solid residue and/or overgrowth in melt domains. The monazite core compositions, with distinctly lower Y, Th and U contents, suggest its formation in equilibrium with garnet. The U–Th–Pb dating of monazite using electron microprobe analysis yielded a c. 200 Ma age for the older cores with low Th, Y, U and high La/Nd ratio, and a c. 160 Ma age for the dominant younger monazite enriched in Th, Y, U and HREE. The older age of c. 200 Ma is interpreted as the timing of UHPM, whereas the younger age of c. 160 Ma as granulite facies/partial melting overprint. Our results suggest that UHPM occurred in Late Triassic to Early Jurassic time, in the framework of collision and subduction of continental crust after the closure of Paleotethys.

Formation and Exhumation of Ultrahigh-Pressure Terranes

The reigning paradigm for the formation and exhumation of continental ultrahigh-pressure (UHP) terranes is the subduction of crust to mantle depths and the return of crustal slices within the subduction channel—all at plate tectonic rates. Additional processes beyond the paradigm are needed to explain the diversity of geological observations gathered from the growing study of UHP terranes—for example, variations in the size, degree of deformation, petrologic evolution, timing of UHP metamorphism, and exhumation rates. Numerical models that evaluate physical parameters in time and space have produced new insights into the formation and exhumation of UHP terranes.

Zircon U–Pb age and geochemical constraints on the tectonic affinity of the Jiaodong terrane in the Sulu orogen, China

Because of tectonic mixture by subduction erosion during Triassic continental collision between the North and South China Blocks, identifying the tectonic affinity of the Jiaodong terrane in the Sulu orogen is a key to recognition of the suture location in east-central China. In this study, zircon U–Pb ages, whole-rock major and trace elements as well as Rb–Sr and Sm–Nd isotopes, and mineral O isotopes were determined for ultrahigh-pressure (UHP) granitic gneiss and enclosed eclogite in the Jiaodong terrane, a petrotectonic unit southeast of the Wulian-Yantai fault. The granitic gneiss is characterized mostly by: (1) the presence of major Neoproterozoic igneous zircons and minor Triassic metamorphic zircons, which yielded the U–Pb ages of 741 ± 6 Ma interpreted as the emplacement age of the protoliths and 234 ± 5 Ma as the time of UHP metamorphism, respectively; (2) negative ɛ Nd( t) values of −7.4 to −3.8 at t = 750 Ma with Nd model ages of 1.73–2.03 Ga; and (3) low δ 18O zircons of −0.4 to 4.1‰. Zircons from the eclogite also yield concordant metamorphic ages ranging from 229 ± 3 to 239 ± 7 Ma, with low δ 18O values of 0.2–5.1‰. Thus, both granitic gneiss and eclogites share the same 18O depletion in their Neoproterozoic protoliths and experienced the same metamorphic event in the Triassic. Zircon U–Pb ages of 1866 ± 34 to 2057 ± 31 Ma were obtained for a few samples, consistent with the presence of Paleoproterozoic crust in the South China Block. In particular, the widespread occurrence of Neoproterozoic igneous protoliths with low δ 18O values is evident in this region, with an outcrop area of over 10,000 km 2. Such Neoproterozoic igneous rocks with low δ 18O are not found in the North China Block, but are considered to be characteristic products of Neoproterozoic rift magmatism along the northern margin of the South China Block. Therefore, the protoliths of UHP metamorphic rocks in the Jiaodong terrane have a tectonic affinity to the South China Block, like those rocks elsewhere along the Dabie–Sulu orogenic belt. Their petrogenesis is explained by the plate-rift model within the framework of tectonic evolution from supercontinental rift to breakup during the middle Neoproterozoic. Despite crustal detachment and tectonic accretion during the Triassic continental collision, the presence of 1.8–2.0 Ga protoliths in the UHP metamorphic zone does not mean that these lithotectonic slices were derived from the continental margin of the North China Block.

SHRIMP U-Pb Dating of Zircon from the Xugou UHP Eclogite, Sulu Terrane, Eastern China

Eclogites, together with garnet clinopyroxenites, occur as lenses within the Xugou garnet peridotite body in the southern Sulu ultrahigh-pressure (UHP) terrane. Combined cathodoluminescence (CL) imaging and SHRIMP U-Pb dating of zircon from two Xugou mafic eclogites provide added constraints on the timing of UHP metamorphism in this area. Zircons from both samples show subrounded to rounded shapes and patchy CL patterns without inherited igneous cores, indicating that they are metamorphic zircons. SHRIMP U-Pb analyses of these zircons yielded apparent U-Pb ages of 214-280 Ma, with a weighted mean age of 237 ± 8 Ma, which is consistent with previous reported UHP metamorphic ages from eclogite pods and country-rock gneisses. The Xugou mafic lenses may have formed by partial melting of the enclosing peridotites in the mantle before subduction (Zhang et al., 2003); then these eclogites, together with the host peridotites, were tectonically emplaced into the subduction zone and subjected to UHP metamorphism at 237 ± 8 Ma.

The onset of India–Asia continental collision: Early, steep subduction required by the timing of UHP metamorphism in the western Himalaya

Ultrahigh-pressure (UHP) rocks in the NW Himalaya are some of the youngest on Earth, and allow testing of critical questions of UHP formation and exhumation and the timing of the India–Asia collision. Initial collision of India with Asia is widely cited as being at 55 ± 1 Ma based on a paleomagnetically determined slowdown of India's plate velocity, and as being at ca. 51 Ma based on the termination of marine carbonate deposition. Even relatively small changes in this collision age force large changes in tectonic reconstructions because of the rapid India–Asia convergence rate of 134 mm/a at the time of collision. New U–Pb SHRIMP dating of zircon shows that Indian rocks of the Tso Morari Complex reached UHP depths at 53.3 ± 0.7 Ma. Given the high rate of Indian subduction, this dating implies that Indian continental crust arrived at the Asian trench no later than 57 ± 1 Ma, providing a metamorphic age for comparison with previous paleomagnetic and stratigraphic estimates. India's collision with Asia may be compared to modern processes in the Timor region in which initiation of collision precedes both the slowing of the convergence rate and the termination of marine carbonate deposition. The Indian UHP rocks must have traveled rapidly along a short, hence steep, path into the mantle. Early continental subduction was at a steep angle, probably vertical, comparable to modern continental subduction in the Hindu Kush, despite evidence for modern-day low-angle subduction of India beneath Tibet. Oceanic slab break-off likely coincided with exhumation of UHP terranes in the western Himalaya and led to the initiation of low-angle subduction and leucogranite generation.

Cogenetic relationship of the Yangkou gabbro-to-granite unit, Su-Lu terrane, eastern China, and implications for UHP metamorphism

The Yangkou gabbro-to-granite complex is a petrotectonic unit of the Su-Lu ultrahigh-pressure (UHP) metamorphic terrane in eastern China. It consists of lherzolites, metagabbro–eclogites, metagranodiorites and deformed granites. Whether or not the felsic members of the complex have undergone UHP metamorphism as is recorded in the metagabbro–eclogites is significant to understanding the relationship between eclogite and host rock. The metagabbro–eclogites have low ε Nd (220 Ma) values (−17 to −12) at the time of UHP metamorphism, indicating that their protoliths are old mafic intrusive rocks emplaced in a continental domain. Geochemical and isotopic studies for the Yangkou unit suggest that the protoliths of the unit (except eclogites) represent a cogenetic magmatic suite, produced by differentiation of a mafic parental magma. The parental magma was derived from melting of a long-term enriched mantle, and was emplaced into a crustal level during continental extension at c . 700–800 Ma. The cogenetic relationship for the protoliths of the unit suggests that the metagranodiorites and deformed granites were also subducted to extreme mantle depths along with the metagabbro–eclogites at c . 220 Ma. The lherzolites, however, are genetically not related to the mafic protoliths; they represent ‘mantle xenoliths’ of tectonic origin.