Isotope-Geochemical Evidence of the Nature of the Protoliths of Diamondiferous Rocks of the Kokchetav Subduction–Collision Zone (Northern Kazakhstan)

—The isotope-geochemical features of diamondiferous metamorphic rocks of the Kokchetav subduction–collision zone (KSCZ) show that both the basement rocks and the sediments of the Kokchetav massif were their protoliths. A whole-rock Sm–Nd isochron from the diamondiferous calc-silicate, garnet–pyroxene rocks and migmatized granite-gneisses of the western block of the KSCZ yielded an age of 1116 ± 14 Ma, while an age of 1.2–1.1 Ga was obtained by U–Pb dating of zircons from the granite-gneiss basement of the Kokchetav microcontinent. Based on these data, we assume that the protoliths of the calc-silicate, garnet–pyroxene rocks and the granite-gneisses of the KSCZ were the basement rocks sharing an initially single Nd source, which was not influenced by high- to ultrahigh-pressure metamorphism (~530 Ma). Therefore, their geochemical features are probably not directly related to ultrahigh-pressure metamorphism. The corresponding rock associations lack isotope-geochemical evidence of partial melting that would occur during ultrahigh-pressure metamorphism, which suggesting that they were metamorphosed under granulite-facies conditions. At the same time, the high-alumina diamondiferous rocks of the Barchi area (garnet–kyanite–mica schists and granofelses), which were depleted to different degrees in light rare-earth elements (REE) and K, have yielded a Sm–Nd whole-rock isochron age of 507 ± 10 Ma indicating partial melting of these rocks during their exhumation stage. The close ɛNd (1100) values of the basement rocks and garnet–kyanite–mica schist with geochemical characteristics arguing against its depletion during high-pressure metamorphism indicate that the basement rocks were a crustal source for high-alumina sediments.

Evidence for ultrahigh-pressure metamorphism discovered in the Appalachian orogen

The Appalachian orogen has long been enigmatic because, compared to other parts of the Paleozoic orogens that formed following the subduction of the Iapetus Ocean, direct evidence for ultrahigh-pressure (UHP) metamorphism has never been found. We report the first discovery of coesite in the Appalachian orogen in a metapelite from the mid-Ordovician (Taconic orogeny) Tillotson Peak Complex in Vermont (USA). Relict coesite occurs within a bimineralic SiO2 inclusion in garnet. In situ elastic barometry and trace-element thermometry allow reconstruction of the garnet growth history during prograde metamorphism. The data are interpreted to indicate garnet nucleation and crystallization during blueschist- to eclogite-facies subduction zone metamorphism, followed by garnet rim growth at UHP conditions of > 28 kbar and > 530 ° C. Results provide the first direct evidence that rocks of the Appalachian orogen underwent UHP metamorphism to depths of > 75 km and warrant future studies that constrain the extent of UHP metamorphism.

Supplemental Material: Evidence for ultrahigh-pressure metamorphism discovered in the Appalachian orogen

Methodological background and petrologic data.<br>

Supplemental Material: Evidence for ultrahigh-pressure metamorphism discovered in the Appalachian orogen

Methodological background and petrologic data.<br>

Constraining the process of intracontinental subduction: implications from petrology and Lu-Hf geochronology of eclogites from the Austroalpine Nappes

&lt;p&gt;The nucleation of subduction zone remains a widely discussed topic in the global tectonics. The prevalent view is that subduction starts within an oceanic plate. However, there is strong evidence that subduction can also be initiated within a continent. To test this hypothesis, we combine petrology, isotope geochronology and thermodynamic phase equilibrium modelling on eclogites from the Austroalpine Nappes of the Eastern Alps.&lt;/p&gt;&lt;p&gt;The high- and ultrahigh-pressure rocks occur in a ~400 km long belt from the Texel Complex in the west to the Sieggraben Unit in the east without remnants of Mesozoic oceanic crust. Garnet growth during pressure increase was dated using Lu-Hf chronometry. The results range between c. 100 and c. 90 Ma, indicating a short period of subduction. Combined with already published data, our estimates of metamorphic conditions indicate a field gradient with increasing pressure and temperature from northwest to southeast, where the rocks experienced ultrahigh-pressure metamorphism. The oldest Cretaceous eclogites (c. 100 Ma) are found in the Saualpe-Koralpe area which comprises widespread gabbros formed during Permian to Triassic rifting. This supports the hypothesis that subduction initiation was intracontinental and localized by a Permian rift. In the Texel Complex two-phased garnets yielded a Variscan-Eoalpine mixed age indicating re-subduction of Variscan eclogite-bearing continental crust during the Eoalpine orogeny. Jurassic blueschist-facies metamorphism at Meliata in the Western Carpathians and Cretaceous eclogite-facies metamorphism in the Austroalpine are separated by a time gap of ~50 Ma and therefore do not represent a transition from oceanic to continental subduction but rather separate events.&lt;/p&gt;

Evolution of geodynamics since the Archean: Significant change at the dawn of the Phanerozoic

A time-series analysis of thermobaric ratios (temperature/pressure [T/P]) for Paleoarchean to Cenozoic metamorphic rocks identified significant shifts in mean T/P that may be related to secular change in the geodynamics on Earth. Thermobaric ratios showed significant (&gt;95% confidence) change points at 1910, 902, 540, and 515 Ma, recording drops in mean T/P, and at 1830, 604, and 525 Ma, recording rises in mean T/P. Highest mean T/P occurred during the Mesoproterozoic, and lowest mean T/P occurred from the Cambrian to the Oligocene. Correlated changes were seen between T/P and global data sets of time-constrained hafnium (Hf) and oxygen (O) isotope compositions in zircon. The range of correlated variation in T/P, Hf, and O was larger during the formation of Rodinia than Columbia. Large changes and a wide range for these variables continued through the Phanerozoic, during which a statistically significant 83 m.y. frequency of T/P excursions recorded the high tempo of orogenic activity associated with the separation, migration, and accretion of continental terranes during the formation of Pangea. Since the early Tonian, the decreasing mean T/P of metamorphism, widespread appearance of blueschist and ultrahigh-pressure metamorphism, and wide fluctuations in Hf and O isotope compositions document a change to the modern plate-tectonic regime, characterized by widespread continental subduction and deeper slab breakoff than in the Proterozoic.

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

The 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 and P–T conditions. 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–T calculations suggest that metapelite may have experienced a clockwise P–T path with peak P/T conditions 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.