Petrography and phase equilibrium modeling of Paleoproterozoic metapelite in the Kuluktag area of Tarim Craton

<p>  The Kuruqtag area, located at the northeastern margin of the Tarim Craton, where the Precambrian metamorphic basement exposed, is ideal for studying the Precambrian geological evolution of the Tarim Craton. Previous zircon U-Pb chronology studies revealed that the metamorphic basement recorded a Paleoproterozoic tectonothermal event and suggested it associates with the Paleoproterozoic Nuna/Columbian supercontinent convergence event. However, the extensive range of metamorphic ages obtained from different studies (ranging from 1750-2000 Ma) and the lack of detailed P-T path corresponding to different metamorphic ages make it difficult to constrain the evolutionary framework of the Tarim craton during the Paleoproterozoic, which in turn affects future comparative regional studies.</p><p>  To constrain the P-T path, in this study, we performed detailed petrography, mineral chemical, and phase equilibrium modeling of metapelite collected from the khondalite series in the western part of the Kuruqtag (a garnet-sillimanite-cordierite-biotite gneiss with metamorphic age ~1850 Ma) and obtained the following results.</p><p>  Through petrographic studies, at least three phases of mineral assemblages can be used to invert the P-T path experienced by the metapelite. They are    M1 (peak metamorphic stage):represented by fine-grained biotite remnant (Bi Ⅰ) + fine-grained plagioclase(Pl Ⅰ) and quartz+ Ilmenite + , occurring as inclusions within the metamorphic garnet, and equilibrated mineral assemblages is: Grt(core) + Bi Ⅰ + Sill + Kfs + Pl Ⅰ + Qz + Ilm. M2 (isothermal depression stage), represented by cordierite occurring in the garnet rim or with spinel in the matrix, inferred equilibrated mineral assemblages is Grt(rim)+Bi Ⅰ +Cd+Kfs+Pl ⅠⅠ+Ilm+Sp.M3 (isothermal depression stage), is marked by the appearance of new growth of biotite(Bi ⅠⅠ) and the conversion of Sill to And.<span> </span></p><p>The P-T conditions for the mineral assemblage evolution (M1 → M3) are constrained by a P-T pseudosection constructed in the Na2O -CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O- TiO2-O2 chemical system. The resulting P-T path is clockwise from the M1 stage (840°C, 4 Kbar) through the isothermal depression path to M2 (840-850°C,5 Kbar) and then through the near-isobaric cooling path to the M3 stage (650°C, 3.5-4 Kbar).</p>

The impact of the Pan-African-aged tectonothermal event on high-grade rocks at Mount Brown, East Antarctica

This study presents monazite and rutile U–Pb and hornblende and biotite 40Ar/39Ar geochronological data for high-grade rocks of the eastern Grenville-aged Rayner orogen at Mount Brown in order to analyse the extent and degree of Pan-African-aged reworking. Monazite from paragneiss yields U–Pb ages of 910 Ma for larger granular grains and 670–630 Ma for smaller globular beads around garnet porphyroblasts or hosted by symplectites. Rutile from leucogneiss yields U–Pb ages of 520–515 Ma. Hornblende and biotite from different rock types yield 40Ar/39Ar plateau ages of 744 and 520–505 Ma, respectively. Combining these results with published zircon U–Pb age data suggests that granulite facies metamorphism occurred at 910 Ma, with a local low-temperature fluid flow event at 670–630 Ma and thermal reworking at 520–505 Ma. The older age of 744 Ma may reflect cooling or partial resetting of the hornblende 40Ar/39Ar system, indicating that Pan-African-aged reworking did not exceed temperatures much higher than the hornblende Ar closure temperature. These data also suggest that the complete isotopic resetting of some minerals may occur without the growth of new mineral phases, providing an example of the style of reworking that is likely to occur in polymetamorphic terranes.

Chapter 1 Introduction to the lithotectonic framework of Sweden and organization of this Memoir

The solid rock geology of Sweden comprises three principal components: (1) Proterozoic and (locally) Archean rocks belonging to the western part of the Fennoscandian Shield; (2) Phanerozoic and (locally) Neoproterozoic sedimentary cover rocks deposited on top of this ancient crust; and (3) the early to mid-Paleozoic (0.5–0.4 Ga) Caledonide orogen. Earlier compilations have applied different principles for the subdivision of the geology in the Fennoscandian Shield and the Caledonide orogen. A uniform lithotectonic framework has been developed here. Crustal segments affected by orogenesis have been identified and their ages determined by the youngest tectonothermal event. Four ancient mountain belts and six orogenies are preserved. Solid rocks outside the orogens have been assigned to different magmatic complexes or sedimentary successions based on their time of formation and tectonic affiliation. This approach allows relicts of older mountain-building activity to be preserved inside a younger orogen – for example, the effects of the Archean (2.8–2.6 Ga) orogeny inside the 2.0–1.8 Ga Svecokarelian orogen and Paleo–Mesoproterozoic (1.7–1.5 and 1.5–1.4 Ga) mountain-building processes inside the 1.1–0.9 Ga Sveconorwegian orogen. Sweden's five largest mineral districts are addressed in the context of this new lithotectonic framework, which forms the architecture to the contents of the chapters in this Memoir.

Chapter 22 Upper and uppermost thrust sheets in the Caledonide orogen, Sweden: outboard oceanic and exotic continental terranes

Three separate stacks of thrust sheets (Köli Nappe Complex) constitute the Upper Allochthon in the Caledonide orogen, Sweden. This thrust complex is dominated by late Cambrian–Ordovician successions deposited in subduction-related, marginal oceanic basins. Magmatic activity at c. 488 Ma (Lower Köli) and c. 492–476 Ma (Middle Köli) is linked to rifted volcanic arcs and Zn–Cu–Fe–(Pb–Au–Ag) sulphide mineralization; serpentinite bodies with talc deposits are also conspicuous. Renewed magmatic activity, both plutonic (Upper and Middle Köli) and mafic volcanic (Middle and Lower Köli), occurred at c. 440–434 Ma during crustal extension. Late Ordovician shallow-marine sedimentation, deepening upwards into an early Silurian succession also prevailed (Lower Köli). Silurian (c. 430 Ma and later) folding, eastwards-vergent thrusting and greenschist or lower amphibolite facies metamorphism preceded upright, orogen-parallel and orogen-transverse open folding. Juxtaposition of an arc-related terrane to an ancient continental margin, comprising slices of gneiss and marble, in the Middle Köli occurred prior to c. 437 Ma and the eastwards-vergent thrusting; remnants of an Ordovician amphibolite facies tectonothermal event are also preserved in the Upper Köli. The tectonic roof to the Köli complex contains amphibolite facies mica schist, gneiss and marble, derived from the Laurentian continental margin, and a major gabbroic pluton (Rödingsfjället Nappe Complex, Uppermost Allochthon).

Rapid extensional unroofing of a granite–migmatite dome with relics of high-pressure rocks, the Podolsko complex, Bohemian Massif

The Podolsko complex, Bohemian Massif, is a high-grade dome that is exposed along the suprastructure–infrastructure boundary of the Variscan orogen and records snapshots of its protracted evolution. The dome is cored by leucocratic migmatites and anatectic granites that enclose relics of high- to ultrahigh-pressure rocks and is mantled by biotite migmatites and paragneisses whose degree of anatexis decreases outwards. Our new U–Pb zircon ages indicate that the leucocratic migmatites were derived from Early Ordovician (c. 480 Ma) felsic igneous crust; the same age is inferred for melting the proto-source of the metapelitic migmatites. The relics of high- to ultrahigh-pressure rocks suggest that at least some portions of the complex witnessed an early Variscan subduction to mantle depths, followed by high-temperature anatexis and syntectonic growth of the Podolsko dome in the middle crust at c. 340–339 Ma. Subsequently, the dome exhumation was accommodated by crustal-scale extensional detachments. Similar c. 340 Ma ages have also been reported from other segments of the Variscan belt, yet the significance of this tectonothermal event remains uncertain. Here we conclude that the 340 Ma age post-dates the high-pressure metamorphism; the high temperatures required to cause the observed isotopic resetting and new growth of zircon were probably caused by heat input from the underlying mantle and, finally, the extensional unroofing of the complex requires a minimum throw of about 8–10 km. We use this as an argument for significant early Carboniferous palaeotopography in the interior of the Variscan orogen.

Crustal geodynamics from the Archaean Bundelkhand Craton, India: constraints from zircon U–Pb–Hf isotope studies

A comprehensive study based on U–Pb and Hf isotope analyses of zircons from gneisses has been conducted along the western part (Babina area) of the E–W-trending Bundelkhand Tectonic Zone in the central part of the Archaean Bundelkhand Craton. 207Pb–206Pb zircon ages and Hf isotopic data indicate the existence of a felsic crust at ~ 3.59 Ga, followed by a second tectonothermal event at ~ 3.44 Ga, leading to calc-alkaline magmatism and subsequent crustal growth. The study hence suggests that crust formation in the Bundelkhand Craton occurred in a similar time-frame to that recorded from the Singhbhum and Bastar cratons of the North Indian Shield.