High-grade Metamorphism Research Articles

Post-1.9 Ga evolution of the south Rae craton (Northwest Territories, Canada): A Paleoproterozoic orogenic collapse system

The Trans-Hudson Orogen (THO), formed from the convergence between the Superior craton and the composite Churchill Upper Plate (CUP), is one of the best-preserved examples of a collisional orogen in the Paleoproterozoic. Similar to modern collision systems such as the Himalayan orogen, it is characterized by a composite upper plate in which terrane accretion established a continental plateau that was tectonically and magmatically active for >100 myr. Our study presents new petrological and geochronological data for four samples collected in three lithotectonic domains of the south Rae craton (one of the CUP terranes). The results presented here allow us to re-define the previously proposed extent of THO reworking in the CUP and afford the opportunity to study and compare the evolution of various fragments that illustrate differing levels of a collapsed plateau in the CUP hinterland. The new data indicate that the south Rae craton locally preserves evidence for burial at 1.855–1.84 Ga with peak metamorphic conditions at approximately 790 °C and 9.5–12.5 kbar followed by rapid cooling and decompression melting (P < 6 kbar) at ca. 1.835–1.826 Ga.These results, which provide important and so far missing Pressure-Temperature-time (P-T-t) constraints on the evolution of the south Rae craton in the Northwest Territories at Trans-Hudson time, coupled with existing regional geochronological and geochemical data, are used to propose an updated model for the post-1.9 Ga THO collision and extensional collapse. Our results reveal that: i) initial thickening in the upper plate started at Snowbird time (ca. 1.94 Ga), then continued via Sask collision (with high-grade metamorphism recorded in the south Rae craton, ca. 1.85 Ga), and ended with Superior collision (ca. 1.83 Ga); ii) the extent of the THO structural and metamorphic overprint in the SW CUP is much broader across strike than previously recognized, and iii) T-t data in the south Rae are indicative of relatively fast cooling rates (8–25 °C/Ma) compared to other known Precambrian orogens. We suggest that the Paleoproterozoic THO represents the first record of a major ‘modern-style’ orogenic plateau collapse in Earth’s history.

Long-lived (ca. 22–24 Myr) partial melts in the eastern Himalaya: Petrochronologic constraints and tectonic implications

Partial melting is thought to profoundly impact the rheology and deformation behavior of the middle crust. Consequently, investigations of the pressure-temperature conditions of metamorphism, rates of heating, and durations of anatexis can provide unique constraints on tectonic processes. The Greater Himalayan Sequence (GHS), in the metamorphic core of the Himalayan orogen, is commonly considered to represent exhumed, anatectic, mid-crust. Here, we present detailed petrological and geochronological analysis of anatectic pelitic schist and felsic paragneiss from the uppermost structural level of the GHS to understand the timing and conditions of Himalayan anatexis. Petrologic analysis indicates that these rocks experienced high-grade metamorphism and partial melting up to peak conditions of ca. 720–745°C and ca. 9.6–10 kbar. Melt volumes of ca. 3% increased slightly during exhumation with nearly constant or slightly decreasing temperature, then decreased as rocks cooled, ultimate crossing the solidus at ca. 5.5 kbar and 700°C. Well-correlated U–Th–Pb ages and trace element data (HREE, Y, and Eu/Eu*) for monazite and zircon require prograde metamorphism and initial partial melting of GHS rocks at ca. 50 and 42–40 Ma, respectively, and crystallization of melts at ca. 24–18 Ma. These data indicate a long-lived (ca. 22–24 Myr) partially molten mid-crust in the eastern Himalayan orogen that formed as much as 10 Myr earlier and lasted 10 Myr longer than numerical models of viscous flow have predicted. Thermal buffering and melt stagnation may reflect feedbacks between thermal structure and shear stress. The change from thermal and mechanical stasis to rapid exhumation and cooling at ca. 24 Ma corresponds with an orogen-wide shift in deformation patterns, and may reflect arrival of mainland India.

Zircon and monazite geochronology from the Rengali-Eastern Ghats Province: Implications for the tectonic evolution of the eastern Indian terrane

The tectonic evolution of the Rengali Province and its eventual juxtaposition to the Eastern Ghats Province has important bearings on the geological evolution of the Eastern Indian terrane. New zircon and monazite age data from the Rengali Province and the northern-northwestern part of the Eastern Ghats are presented in this study to trace this evolutionary history. Monazite U-Th-total Pb data from the paragneissic rocks of the eastern Rengali Province show a metamorphic age of 2775 ± 18 Ma while an older age of 2943 ± 35 Ma from the same rock probably suggests an older metamorphic/magmatic event. Zircon U-Pb (LA-ICPMS) data from the northern part of the Eastern Ghats show 1230 ± 21 Ma and 1220 ± 9 Ma ages that we interpret as a major phase of high-grade metamorphism of the basement. Paragneissic rocks from the northwestern margin of the Eastern Ghats yields monazite ages of 966 ± 21 Ma and 555 ± 12 Ma respectively from the core and rim parts of monazite grains. Similar ages of 966 ± 25 Ma and 540 ± 12 Ma are reported from paragneissic rock occurring at the contact of Rengali Province and the Eastern Ghats. This younger (~555–540 Ma) age likely correlates to the amphibolite facies reworking of the granulitic lower crust which coincides with the emplacement of nepheline syenite at 556 ± 28 Ma (zircon U-Pb data) and the contact metamorphism of the ultramafic granofels at 553 ± 18 Ma (monazite data). Nepheline monzosyenite veins intruded the gneissic nepheline syenite at 506 ± 9 Ma (zircon U-Pb data). Emplacement of the monzosyenite veins within the felsic gneiss country at the northwestern margin of the Eastern Ghats at 490 ± 3 Ma (zircon U-Pb data) marks the last thermal imprint in response to large-scale shear-induced deformation at the northern/northwestern contacts of the Eastern Ghats. We infer that the Neoarchean (ca. 2943–2775 Ma) events possibly resulted from the ensuing convergent tectonics driven by lithospheric peeling (peel-back convergent tectonics). The Eastern Ghats and its Antarctic counterpart juxtaposed with the Rengali Province during ca. 1000–900 Ma and become a part of the Eastern Indian terrane. The Ediacaran-Cambrian (ca. 556–490 Ma) events imply the reactivation of the deep crustal Tonian-age shear systems in a transpressional tectonic setting.

Petrologic and geochronological constraints on the polymetamorphic evolution of the collisional granites, Araçuaí Orogen (SE Brazil).

Collisional granites of the Araçuaí Orogen, southeastern Brazil, record petrological and geochronological evidence for multiple crustal melting during the orogeny evolution. U-Pb zircon data indicate that these granites crystallized at 586 ± 2 M.y. High-grade metamorphism (M1) involved partial melting by fluid-absent reactions that produced the first generation of garnet in temperatures of approx. 750°C. Preservation of the mineral assemblage A1 (garnet-biotite-plagioclase-K-feldspar-quartz-ilmenite-melt) indicates that most of the generated melt was lost from these rocks at or near peak metamorphic conditions. A second metamorphic event (M2) is characterized by growth of a second generation of garnet in preserved A2 assemblage (garnet-sillimanite-biotite-plagioclase-K-feldspar-quartz-ilmenite-melt). Mineral equilibria modeling constrains conditions of M2 metamorphism to 713-729 °C and 6.2-7.3 kbar. Retrograde assemblage (A3) records equilibrium conditions at 610-660 °C. The Hf isotope composition indicates significant crustal contribution to the genesis of the collisional granites. The elevated geotherms in thickened crust provide enough heat for the M1 event at 562 ± 2 M.y. Subsequent heating probably associated to the transfer of mantle heat to the crust during the extensional thinning and gravitational collapse of the orogen lead to the M2 event at 526 ± 4 M.y. This event is concomitant to the emplacement of the post-collisional magmas in the orogen.

The complexities of Mesoarchean to late Paleoproterozoic magmatism and metamorphism in the Qixia area, eastern North China Craton: Geology, geochemistry and SHRIMP U-Pb zircon dating

Qixia is a typical area of early Precambrian basement in eastern Shandong Province, eastern North China Craton. Many TTG (tonalite-trondhjemite-granodiorite) assemblages were once considered to be supracrustal rocks (the Jiaodong Group), and the formation ages of the rocks have only been determined in a few outcrops as shown on the early geological map of the area. We carried out geological mapping, geochemical study and SHRIMP U-Pb zircon dating in order to determine the temporal and spatial distribution and origins of the TTG rocks. In the newly compiled geological map (1:50,000), the main rock types of the Archean basement are ∼2.9 Ga, ∼2.7 Ga and ∼2.5 Ga tonalitic gneisses with local occurrences of trondhjemitic gneisses, granodioritic gneisses, (quartz) dioritic gneisses and meta-gabbro showing the same age range. Supracrustal rocks with ages of ∼2.9 Ga and ∼2.5 Ga are locally identified. All rocks broadly extend in a NW-SE direction as a result of strong tectonothermal events of the late Neoarchean and late Paleoproterozoic. Although the late Paleoproterozoic tectonothermal event strongly influenced all 2.7 to 2.9 Ga rocks in the area, metamorphic zircon ages are not widely recorded in these rocks because the high-grade metamorphism at ca. 2.5 Ga caused the older 2.7 to 2.9 Ga rocks to become relatively dry systems. The three generations of TTG rocks are similar in major element composition, characterized by high Na2O and low K2O, except for the late Neoarchean granodioritic gneisses, which locally occur and are relatively high in K2O. All the TTG rocks of different ages commonly have zircon O isotopic compositions within the range determined by Valley and others (2005) for Archean magmatic zircon. The ∼2.9 Ga TTG rocks show large Sr/Y and La/Yb variations and depletion in whole-rock Nd and zircon Hf isotopic compositions. The ∼2.7 Ga TTG rocks are similar in Nd-Hf isotopic compositions to the ∼2.9 Ga TTG rocks but have low Sr/Y and La/Yb ratios. The ∼2.5 Ga TTG rocks are similar in trace element composition to the ∼2.9 Ga TTG rocks, showing large variations in Sr/Y and La/Yb ratios. They can be further subdivided into two types in terms of Nd-Hf isotopic compositions with the depleted type mainly including tonalitic gneisses [eNd(t) = +1.86 to +4.59, eHf(t) = +1.0 to +8.7] and the enriched type including trondhjemitic and granodioritic gneisses [eNd(t) = −2.38 to −0.06, eHf(t) = −1.6 to −2.9]. It is concluded that the ∼2.9 Ga TTG rocks were formed in an oceanic environment (oceanic plateau or intra-ocean subduction), and the 2.7 Ga TTG rocks were formed by mantle underplating that resulted in partial melting of lower crustal mafic rocks under relatively low pressure conditions. More ancient continental materials played a role in the ∼2.5 Ga magmatic process, but more work is required to determine the tectonic environment (underplating or arc magmatism).

The geology of the Clifden district, Connemara Co. Galway, Ireland and present understanding of Connemara geology

The Clifden district, illustrated with an accompanying new detailed 1:12,500 geological map and fold trace map, includes most of the stratigraphy of the Connemara Dalradian and is used to summarise present understanding of the older geological history of Connemara in the light of new structural work and recent age determinations. The Grampian metamorphism peaked with sillimanite formation during and after the late D2 syntectonic intrusions of metagabbros at 475–470Ma and the formation of the major D2 Derryclare fold which was then repeatedly folded by numerous major D3 folds synchronous with the injection in the south of the 467Ma Quartz diorite gneiss suite, maintaining the high-grade metamorphism. Late D3 saw the general initiation of cooling and uplift, the latter pronounced in eastern Connemara above and around the subterranean gathering of the Oughterard Granite magma, plus movement on the late D3 Renvyle-Bofin slide, generating an area of lower pressure metamorphism with cordierite and andalusite not found in the Clifden area.

Late Ediacaran lateral-escape tectonics as recorded by the Patos shear zone (Borborema Province, NE Brazil)

Lateral escape tectonics mediated by strike-slip fault zones are an efficient mechanism of rock deformation in the continental crust of collision zones. The Borborema shear zone system, which shows dimensions comparable to the Cenozoic extrusion of Indochina, defines a lateral escape setting of the Brasilano orogeny in Northeastern Brazil. In this paper, the timing of high-grade metamorphism and compositions of the terranes involved in the shear deformation were investigated. The Patos shear corridor deforms the Siderian to Neoarchean rocks of the Granjeiro Complex that, in turn, form the basement of Seridó-Lavras da Mangabeira metapelites. U-Pb zircon ages and Sm-Nd whole-rock isotopic compositions indicate that the 2.80 – 2.35 Ga basement sequences mainly include juvenile material, whereas zircons from synkinematic migmatites indicate that the partial melting occurred in the Late Ediacaran (ca. 565 Ma). Cooling rates provided by 40Ar/39Ar range from 12 to 17°C/Ma, indicating a differential shear zone exhumation, in agreement with a transpressive setting. The presence of allochthonous Siderian sequences dismembered along the shear zone suggests that the Seridó-Granjeiro corridor defines a major tectonic boundary connected to the collisional front defined by the convergence of São Francisco-Congo and Amazonian cratons.

The geology of the Clifden district, Connemara Co. Galway, Ireland and present understanding of Connemara geology

The Clifden district, illustrated with an accompanying new detailed 1:12,500 geological map and fold trace map, includes most of the stratigraphy of the Connemara Dalradian and is used to summarise present understanding of the older geological history of Connemara in the light of new structural work and recent age determinations. The Grampian metamorphism peaked with sillimanite formation during and after the late D2 syntectonic intrusions of metagabbros at 475–470Ma and the formation of the major D2 Derryclare fold which was then repeatedly folded by numerous major D3 folds synchronous with the injection in the south of the 467Ma Quartz diorite gneiss suite, maintaining the high-grade metamorphism. Late D3 saw the general initiation of cooling and uplift, the latter pronounced in eastern Connemara above and around the subterranean gathering of the Oughterard Granite magma, plus movement on the late D3 Renvyle-Bofin slide, generating an area of lower pressure metamorphism with cordierite and andalusite not found in the Clifden area.

Reworked early Precambrian basement of the Dunhuang Block (NW China) as revealed by titanite U–Pb dating

The Dunhuang Block in the Central Asian Orogenic Belt (CAOB) consists of Archean to Paleoproterozoic crystalline basement, along with Paleozoic complexes that are related to the evolution of the CAOB. A current main controversy concerns whether the whole Dunhuang Block has experienced the Paleozoic tectonothermal events. Titanite, a common U–bearing accessory mineral in igneous and metamorphic rocks, is susceptible to magmatic and metamorphic processes, and thus tends to record relatively late tectonothermal events for metamorphic rocks. In this study, we conducted LA–ICPMS U−Pb dating on titanites from the Archean to Paleoproterozoic rocks in the Dunhuang Block. The titanites are likely metamorphic in origin and yielded U–Pb ages of 409−309 Ma. These ages can be divided into two major groups at 409−390 Ma and 354−309 Ma, corresponding to an early-stage retrograde of high–grade metamorphism and a later-stage magmatism, respectively. Overall data and regional geology indicate that the whole Dunhuang Block has been intensively reworked by the Paleozoic tectonothermal events, leading to metamorphic and magmatic overprints of the early Precambrian basement rocks during the evolution of the CAOB in the Paleozoic. Therefore, the Dunhuang Block should not be considered as an independent Paleozoic orogenic belt. More importantly, our study demonstrates that titanite U–Pb dating can be used as a potential geochronologic tool to reveal late tectonothermal events of the reworked Precambrian metamorphic terranes.

High Ba–Sr adakitic charnockite suite from the Nagercoil Block, southern India: Vestiges of Paleoproterozoic arc and implications for Columbia to Gondwana

The Nagercoil block is the southernmost crustal segment of the Southern Granulite Terrane (SGT) in India and is mainly composed of charnockitic rocks and felsic gneisses (charnockite suite). In this study, we present petrologic, geochemical, zircon U–Pb, REE, and Hf isotopic studies on the charnockites and leucogneiss from the Nagercoil block. Based on field investigations and petrologic studies, the charnockites can be divided into garnet-bearing and garnet-absent anhydrous granulite facies rocks with orthopyroxene. The charnockites and leucogneiss show transition from adakites to non-adakitic magmatic rocks, with enrichment in LREEs (light rare earth elements) and LILEs (large ion lithophile elements), and depletion in HREEs (heavy rare earth elements) and HFSEs (high field strength elements). Some of the charnockites and the leucogneiss show typical HSA (high silica adakite) characters, (high SiO2, Al2O3, Ba–Sr, La/Yb, and Sr/Y). The HSA is considered to have formed from the interaction of slab derived melts and peridotitic mantle wedge. The high Ba–Sr features were possibly inherited from subducted oceanic crust melting under high thermal gradient during Precambrian. The magmas were underplated and subjected to fractional crystallization. Zircon grains from the charnockite and leucogneiss show zoned magmatic cores surrounded by structureless metamorphic rims. Magmatic zircon grains from the charnockites show ages ranging from 1983 ± 8.8 Ma to 2046 ± 14 Ma, and the metamorphic domains show an age range of 502 ± 14 Ma to 547 ± 8.7 Ma. Zircon from the leucogneiss yielded magmatic and metamorphic ages of 1860 ± 20 Ma and 575.6 ± 8.8 Ma. Both charnockites and leucogneiss show two prominent age peaks at 1987 Ma and 568 Ma. The REE data of the zircon grains show LREE depletion and HREE enrichment, with the metamorphic grains showing more depletion in HREE. Zircon Hf isotopic data of the magmatic cores of zircon grains from the charnockite yielded εHf(t) values from −1.17 to 0.46 with TDM and TDMC and age peaks at 2392 Ma and 2638 Ma, suggesting Neoarchean to Paleoproterozoic juvenile sources. We suggest that the high Ba–Sr adakitic charnockite suite from the Nagercoil block formed in a Paleoproterozoic magmatic arc setting during the assembly of the Columbia supercontinent, and underwent high-grade metamorphism associated with the amalgamation of the Gondwana supercontinent during the late Neoproterozoic–Cambrian. Our study provides new insights into the vestiges of Columbia fragments within the Gondwana assembly with two distinct cycles of crustal evolution.

Neoproterozoic crustal growth in southern Malawi: New insights from petrology, geochemistry, and U–Pb zircon geochronology, and implications for the Kalahari Craton–Congo Craton amalgamation

The southern part of the Malawi Basement Complex is regarded as being part of the Mozambique Belt, which was formed by a series of collisions between Archean cratons and/or Proterozoic magmatic arcs during Neoproterozoic to Cambrian time. The region is particularly important because of its location near the junction between the East-African (ca. 600–550 Ma) and Kuunga (ca. 560–530 Ma) orogens, which means that complex magmatic or metamorphic episodes may have been recorded in high-grade metamorphic rocks in the region. Here we report on new petrological, geochemical, and zircon U–Pb age data on felsic to mafic orthogneisses as well as on a syn-tectonic syenite from the Lilongwe–Zomba–Blantyre area in southern Malawi, with a view to evaluating the timing and pressure–temperature (P–T) conditions of the collision events as well as the protolith formation. The geochemical data on the felsic orthogneisses suggested magmatic-arc signatures, whereas the mafic orthogneisses had both magmatic-arc and non-arc signatures. Metamorphic P–T estimates for the orthogneisses based on conventional geothermometry and phase-equilibria modeling indicated high-pressure granulite-facies conditions of 9.1–11.6 kbar and 840 °C–930 °C. Zircons obtained from charnockite, biotite gneiss, and garnet-bearing mafic granulite yielded magmatic ages of 1011–808 Ma, 1013 Ma, and 1076 Ma, respectively, suggesting the presence of Stenian–Tonian crustal growth related to arc magmatism. Ediacaran (573 Ma) intrusion of granitic rock was also inferred from magmatic zircons found within a granitoid gneiss. The timing of the high-grade metamorphism was constrained by the ages of the zircon rims and found to be 569–557 Ma, thus coinciding with the intrusion of syn-tectonic syenite at 562 Ma. The results of this study suggest that the southern part of Malawi is composed of remnants of the Stenian–Tonian magmatic arc, which were metamorphosed together with surrounding metasediments around 560 Ma. The Ediacaran high-grade metamorphism is consistent with the age of the Zambezi Belt located immediately southwest of the Malawi Basement Complex. Both the Lilongwe–Zomba–Blantyre area and the Zambezi Belt may correspond to a single suture zone formed by the latest Neoproterozoic collision between the Kalahari and Congo Cratons.

40Ar/39Ar dating of hornblende and U-Pb dating of zircon in the Aketashitage orogen, NW China: Constraints on exhumation and cooling in the Paleoproterozoic

The Paleoproterozoic Aketashitage orogenic belt lies in between the two large accretionary orogenic systems, i.e., Paleo-Asian ocean and Paleo-Tethys ocean systems, in northwest China. Previous geochronologic studies reveal widespread Paleoproterozoic orogeny-related magmatism and high-grade metamorphism in this area, determined by the U-Pb dating of zircon and monazite. In this paper, 40Ar/39Ar dating of hornblende of three representative amphibolite samples yielded cooling ages of ∼1.88–1.85 Ga. Comparison of the zircon/monazite U-Pb and hornblende 40Ar/39Ar ages suggests that these amphibolite samples record different exhumation and cooling processes. These new geochronologic data further confirm that the Aketashitage orogenic belt only records Paleoproterozoic metamorphism but no overprint of Paleozoic thermal event. It is thus inferred that the Aketashitage area might act as a continental fragment/terrane which was passively collaged onto the ambilateral Paleozoic orogens but was not involved in the Paleozoic subduction.

Structural analysis and metamorphism of the Barlow Fault Zone, Chibougamau area, Neoarchean Abitibi Subprovince: implications for gold mineralization

Faults and deformation zones are important features of Archean terranes because of their significance for structural evolution and the formation of large gold districts. In the Chibougamau area, northeastern portion of the Abitibi Subprovince, an Apogee Metal Earth seismic reflection survey identified an association between an exceptional shallow-dipping subsurface reflector zone and the Barlow Fault Zone visible at the surface. This fault zone is located adjacent to the Opatica Subprovince and shares many characteristics with well-documented major gold-bearing faults in the more gold-endowed southern portion of the Abitibi Subprovince. This study aimed to reconstruct the kinematic evolution of the Barlow Fault Zone, determine its position within the structural setting of this section of the Abitibi Subprovince, and evaluate its importance for gold potential in the northern part of the Chibougamau area. The fault zone can be divided into three segments that differ markedly in dip. Structural reconstruction and field observations are compatible with a reverse south-over-north movement related to a ductile north–south shortening event, which culminated with amphibolite metamorphism. A late normal movement is deduced at the final stage. Geothermobarometers indicate peak metamorphism conditions of 550 ± 50 °C and 6 ± 1.2 kbar. Results from this study suggest that amphibolite facies metamorphism covers a much wider area within the Chibougamau region than previously documented. The Barlow Fault Zone shares similar geometric characteristics and evolutionary history with other gold-bearing structures in the Abitibi Subprovince, but ultimately it was unable to provide optimal conditions for channelling fluid and precipitating gold. High-grade metamorphism and the limited crustal extent of the fault likely prevented the tapping of deep metamorphic fluid and magma reservoirs. The Barlow Fault Zone is interpreted as a back-thrust fault that belongs to a more extensive south-dipping fault system encasing juxtaposed tectonic slivers. This system, with amphibolite facies metamorphism, is a defining feature of the northern portion of the Chibougamau area and developed during the accretion between the Opatica and Abitibi subprovinces.

Geological subdivision of the Lützow–Holm Complex in East Antarctica: From the Neoarchean to the Neoproterozoic

We summarize U–Pb age data for the Lützow–Holm Complex (LHC) in East Antarctica and propose the following geological subdivisions based on protolith ages, along the coast of Dronning Maud Land from southwest to northeast: the Innhovde Suite (INH, 1070–1040 Ma) composed mainly of felsic orthogneiss; the Rundvågshetta Suite (RVG, 2520–2470 Ma), mostly felsic orthogneiss with minor mafic and metasedimentary gneisses; the Skallevikshalsen Suite (SKV, 1830–1790 Ma), felsic to mafic orthogneiss with abundant dolomitic marbles, calc-silicates and other metasediments; the Langhovde Suite (LHV, 1100–1050 Ma), mostly felsic orthogneiss with minor mafic and calc-silicate gneisses; the East Ongul Suite (EOG, 630 Ma), with various orthogneisses and metasediments; and the Akarui Suite (AKR, 970–800 Ma) with diverse orthogneisses and paragneisses. The oldest crustal components of the LHC lie in the southern part of Lützow–Holm Bay, and consist of late Neoarchean and Paleoproterozoic protoliths to charnockites and enderbites that dominate the Rundvågshetta and Skallevikshalsen Suites. This older domain is surrounded by gneisses and granulites with late Mesoproterozoic to early Neoproterozoic protolith ages, including the Innhovde Suite and the Langhovde Suite. The Akarui Suite contains diverse orthogneisses with Neoproterozoic protoloiths, and the youngest unit is the East Ongul Suite with protolith ages of ~630 Ma. Cape Hinode, located geographically within the Akarui Suite, underwent high-grade metamorphism at ~960 Ma that is much older than that which produced the gneisses and granulites of the surrounding LHC (~600–520 Ma). Cape Hinode is therefore exotic, independent of the surrounding LHC, and defined as the “Hinode Block”. The boundaries proposed in this paper are largely consistent with those inferred from magnetic anomalies, gravity anomalies, and bedrock topographical data.

Zircon and monazite geochronology in the Palmer zone of transpression, south-central New England, USA: Constraints on timing of deformation, high-grade metamorphism, and lithospheric foundering during late Paleozoic oblique collision in the Northern Appalachian orogen

Abstract Middle to late Paleozoic high-angle (45°) oblique convergence between Laurentia and composite Avalon terranes resulted in crustal shortening of ∼5:1 across the Central Maine and Bronson Hill zones of the southern New England Appalachians (USA). The Palmer zone of transpression illustrates the midcrustal expression of magmatism, metamorphism, and ductile deformation that developed in response to oblique convergence in the apparent absence of subduction. Secondary ion mass spectrometry zircon U-Pb and monazite Th-Pb ages, supplemented by zircon laser-ablation–inductively coupled plasma–single-collector mass spectrometry U-Pb ages, expand the geochronology constraining the evolution of the Palmer zone of transpression system and potential models for transcurrent collisional tectonics. Ordovician, Silurian, and Early Devonian plutons, and regional high-grade metapelitic country rocks that comprise the pre-transpressional crustal infrastructure are the same age as lithologic equivalents to the north and northeast along the orogen that did not experience high-angle oblique convergence. All plutons within the Palmer zone of transpression and adjoining areas were deformed (flattened, boudinaged, attenuated, folded, and/or ductilely sheared) by the regional transpression system. The most widespread magmatic event in the study area, which is not observed elsewhere in the Northern Appalachians, is intrusion of a diorite-tonalite suite at 370–360 Ma, the oldest rocks of which contain granulite-facies mineral assemblages and all of which are deformed. Leucopegmatites that intrude pelitic paragneisses, which likely formed in response to high-grade metamorphism and which are all deformed, are 370–355 Ma in age. All metapelitic paragneisses contain foliations and lineations that are synkinematic with retrograde garnet + K-feldspar → sillimanite + biotite assemblages and fabrics that formed during vertical and lateral crustal escape in response to extreme shortening. Contemporaneous diorite-tonalite magmatism and regional high-grade metamorphism are interpreted to reflect regional-scale heating of the crust preceding inception of the transpressional system. Transpressional deformation of both plutons and paragneisses indicates shortening commenced after crustal thermal conditions peaked (ca. 355 Ma). Monazite in paragneiss, the formation of which in most samples is linked texturally to formation of transpressional fabrics, yielded a continuum of Th-Pb ages of ca. 360–330 Ma, indicating transpression continued after peak thermal conditions. The extreme crustal shortening of the Bronson Hill and Central Maine zones from Maine to southern New England, pre-transpressional magmatism, and high-grade metamorphism overprint an Acadian (ca. 400–370 Ma) magmatic-metamorphic infrastructure that occurred in the absence of subduction. The transpressional system exhibits all predicted thermal, magmatic, deformation, metamorphic, and exhumation/erosion characteristics of mantle lithospheric foundering (delamination, detachment, or drip) in response to extreme lithospheric shortening and vertical stretching at ca. 375–370 Ma, leading to advection of heat to the lower continental crust and attendant magmatic and metamorphic responses over the time span 370–355 Ma.

Distribution of the Pan‐African Domains in East Antarctica and Adjacent Areas

AbstractThe Pan‐African event is widely distributed in East Antarctica craton. Many terranes or outcrops of the craton bear more or less signs of the event. From characteristics of the shear zones, granites, pegmatites, feature and time of high grade metamorphism and detrital zircon ages peaks of the downflowing sediments from the plateau, the Pan‐African event in the East Antarctica and adjacent areas in the Gondwana reconstruction, like SE Africa, southern India and SW Australia, wasdistributed as special zones or areas in many localities, including both the coastal regions and interior of the East Antarctica. In geochemistry, the granites are generally anorogenic, ocassionally with some gabbros or dolerite dykes, showing sign of bi‐modal feature. The water or fluid available along the shear zones were responsible for retrogression of the earlier, e.g., Grenville age, high grade outcrops to later Pan‐African amphibolite facies metamorphism. Meanwhile, the Pan‐African event has influenced most isotopic systems, including the U‐Pb, Sm‐Nd, Rb‐Sr and Ar‐Ar systems, giving younger apparent ages. Manifestation of the Pan‐African event is distributed from possibly locally granitic magmatism, to wider medium‐high grade metamorphism, and mostly widespread in resetting for some isotope systems, suggesting the prevailing thermal effect.Before Gondwana formation, local depressions in the East Antarctica could be filled with sediments, implying the initial breakup period of the Rodinia. The later Pan‐Gondwana counterrotating cogs shaped the interstitial fold belts between continent blocks and formed a set of shear zones. The mafic underplating in the Gondwana may be responsible for the widespread granites, pegmatites and more or less isotopic resetting due to strong thermal effect from the deep. That is, the Pan‐African event is a possible response of the plate movement surrounding the continent swarms in the non‐stable interior of theyet consolidated Gondwana. The Pan‐African event may be an overwhelmingly extensional and transcurrent tectonics in mechanism.

QUANTIFICAÇÃO DA DEFORMAÇÃO EM MIGMATITOS METATEXITOS ESTROMÁTICOS E ROCHAS ASSOCIADAS DO COMPLEXO ATUBA, PORÇÃO LESTE DO ESTADO DO PARANÁ

RESUMO – O Complexo Atuba aflora na porção leste do Estado do Paraná, segundo uma faixa alongada de direção NE-SW e compreende a parte sul do Cinturão Ribeira. Este trabalho tem como objetivo principal elucidar os processos deformacionais do Complexo Atuba. A investigação foi desenvolvida por meio de investigação de campo, petrografia e análise estrutural quantitativa e, complementa vários estudos tectônicos já realizados anteriormente. Os migmatitos metatexitos estromáticos possuem médio a alto grau metamórfico e são compostos por neossomas residuais tonalíticos intercalados com leucossomas graníticos ou granodioríticos-tonalíticos e melanossomas máficos. Muitas vezes podem ser classificados como pertencentes à série dos milonitos. Observam-se também anfibolitos, xistos, gnaisses granulíticos, diatexitos schollen, quartzitos e granitoides. Dois planos de foliação, ambos gerados por cisalhamento dúctil, foram identificados: o primeiro é caracterizado pela foliação Sn-1 associada à tectônica de cavalgamento (Dn-1) e o segundo pela foliação Sn produzido por tectônica transpressiva sinistral (Dn). Os resultados confirmam que a última fase foi acomodada pela partição da deformação. A quantificação da deformação foi desenvolvida por intermédio dos métodos de Fry, Polar e Rf/φ, com base na análise de cristais de quartzo e feldspatos em 26 lâminas delgadas. As amostras revelaram razões de deformação similares para todos os métodos utilizados nos planos XZ e YZ. Os elipsoides possuem forma oblata e correspondem ao campo do achatamento aparente.&#x0D; Palavras-chave: análise estrutural, migmatitos, transpressão.

Deformation history of the Qianlishan Complex, Khondalite Belt, North China: Structures, ages and tectonic implications

The Qianlishan Complex is a multiply deformed granulite terrane of the Khondalite Belt that represents a Paleoproterozoic continent-continent collisional orogen in the Western Block of the North China Craton. Two major stages of deformation (D1 and D2) were identified in the Qianlishan Complex. D1 mainly produced NWW-trending overturned to recumbent intrafolial isoclinal folds F1, sub-horizontal penetrative foliations S1, NNE-SSW mineral lineations L1 associated with top-to-the-SSW thrust shear, reflecting crustal thickening due to the NNE-SSW compression. D1 was developed with the prograde (M1) and peak (M2) granulite-facies metamorphism. D2 successively superposed the D1 structures and was characterized by NWW-SEE-trending doubly plunging open to tight upright folds F2 and minor sub-vertical axial planar foliations/cleavages S2. D2 was accompanied by post-peak near-isothermal decompression (M3), probably related to exhumation following the crustal thickening. SIMS U–Pb zircon ages of deformation-related leucocratic dykes demonstrate that D1 broadly occurred at 1976–1936 Ma and D2 subsequently happened at 1936–1854 Ma. These results indicate that the Khondalite Belt underwent long-term (~100 Myr) polyphase deformation and high-grade metamorphism associated with magmatism. Timing constraints and kinematic compatibility suggest that the deformation sequence resulted from the NNE-SSW collision between the Yinshan and Ordos Blocks to form the Western Block at ~1.95 Ga.

Young and Old Granulites: A Volatile Connection

Granulite facies metamorphism of the lower crust has decreased in scale since the Late Archean, but many of its definitive features have persisted: (1) Punctuated, sometimes relatively short-lived, episodes of high-grade metamorphism. These are recorded, in favorably simple cases, by discrete growth rims on zircons. (2) A consistent age gap of a few to several tens of millions of years between juvenile magmatism (crustal accretion) and high-temperature metamorphism. The secondary thermal pulse is an event distinct from primary crustal accretion. (3) Involvement of mineralizing pore fluids of lowered H2O activity, that is, with high CO2 and saline concentrations. Very high oxidation states of some granulites implicate sulfur as an important fluid component. (4) Transcurrent faulting as a conspicuous feature of synmetamorphic deformation. This gives rise to characteristic transposed foliation and lineation. (5) Emplacement of coeval postorogenic K-rich granites at midcrust levels. These features can be rationalized by concepts of modern plate tectonics. High-angle plate collision is succeeded by orogen-parallel transport. This change of plate motion necessarily detaches the underthrust portion of the lithosphere, liberating asthenospheric melts and/or fluids in a postorogenic resurgence. A generation of volatile-rich mafic magmas invades the continental margin; high CO2 and halogen contents cause outgassing and freezing of the magmas at depth. Liberated volatiles effect granulite facies metamorphism by leaching H2O and lithophile elements, importantly K, and transporting these components and heat upward. Extensive melting of the lower crust is inhibited by the low H2O activity of saline-carbonic pore fluids at high pressure. Melting of orthogneiss and supracrustal rocks occurs at midcrust levels by increase of H2O activity as pressure on alkali chloride solutions falls below 0.6–0.5 GPa. The foregoing hypothesis is an alternative to the classical view that granite results from fluid-absent partial melting of, and extraction from, the lower crust, leaving granulites.

The earliest Cambrian UHT metamorphism in the Qaidam block, western China: A record of the final assembly of Greater Gondwana?

The final assembly of Gondwana, known as the late Pan-African orogeny, is characterized by Ediacaran–early Cambrian ultrahigh temperature (UHT) metamorphism, which is widely identified within reconstructed East Gondwana. This distinctive feature likely provides a reliable criterion for identifying new Gondwanan terranes that lack paleo-geomagnetic data. Here we present zircon U–Pb geochronology and phase equilibria calculations for a variety of granulite types newly recognized from western Qaidam, China, which provide the first evidence that the Qaidam block, at least western Qaidam, experienced high-grade metamorphism in excess of 900 °C before/at 540–520 Ma. These UHT metamorphic rocks, similar to many well-known Pan-African UHT metamorphic terranes, is inferred to evolve along a clockwise P–T path that is usually related to collisional orogens. Comparison between new metamorphic zircon U–Pb ages from western Qaidam and the published age data from the UHT metamorphic terranes within East Gondwana suggests that the UHT metamorphic rocks found in western Qaidam similarly records the final assembly of Gondwana. Although the exact paleo-geographical location of the Qaidam block during the Gondwana period is unknown yet because of lacking paleo-geomagnetic data, new Pan-African UHT metamorphic record found in western Qaidam indicates, for the first time, that the Qaidam block is a Gondwanan terrane that split from this semi-supercontinent after the Pan-African orogeny.