Greenschist Facies Metamorphism Research Articles

Constraints on a long-lived Paleoproterozoic metamorphic process in a middle- to lower-grade metamorphic complex, Songshan area, southern North China craton: Evidence from minor- and trace-element thermometry and geochronology

The Dengfeng Complex and Songshan Group comprise a middle- to lower-grade metamorphic complex with abundant lithological associations in the Songshan area (central Henan, China), representing an excellent opportunity to investigate the thermal history of the North China craton. The polymetamorphic Dengfeng Complex is a typical granite-greenstone belt that records a late Paleoproterozoic metamorphic event (ca. 1.95−1.80 Ga) overprinting on a late Neoarchean metamorphic event (ca. 2.51−2.41 Ga). In contrast, the Songshan Group preserves well-developed stratigraphic sequences and sedimentary structures with greenschist-facies metamorphism. Numerous studies have focused on the late Neoarchean metamorphic event from the Dengfeng Complex, whereas the thermal evolution of ca. 1.95−1.80 Ga units from the Dengfeng Complex and Songshan Group has been largely overlooked. Quartzite of the Songshan Group unconformably overlies schists of the Dengfeng Group, and they show coherent NNE-SSW−striking schistosity structures with dip angles of ∼50°−65°. Here, the metamorphic temperatures from the Dengfeng Group were constrained to be ∼510−550 °C by the Ti-in-biotite geothermometer. The Ti-in-quartz and Zr-in-rutile geothermometers record metamorphic temperatures of ∼480−550 °C or 520−555 °C for the Songshan Group, respectively. Laser ablation−inductively coupled plasma−mass spectrometry U-Pb dating of monazite and rutile constrains the timing of metamorphism of the Songshan Group to be ca. 1.94−1.83 Ga, while zircon constrains the timing of metamorphism of the Dengfeng Complex to be ca. 2.53 Ga and ca. 1.95−1.87 Ga. By integrating geochronologic data from the Dengfeng Complex and Songshan Group, the ca. 2.53 Ga metamorphic age of amphibolite may be connected with Neoarchean subduction-accretion processes, whereas the consistent temperature obtained in this study ca. 1.95−1.83 Ga indicates that the Songshan area may have experienced a long-lived metamorphic event, which could have resulted from the final collision between the Western and Eastern blocks of the North China craton in the late Paleoproterozoic. The persistence of a Paleoproterozoic hot orogen for millions of years was probably the norm in the North China craton, which provides new insights into the tectonic-thermal evolution in the southern North China craton.

Unveiling the connection between lode gold mineralization and the metamorphic core complex evolution from the large Yangzhaiyu gold deposit, North China Craton

The Xiaoqinling gold province, located in the Neoarchean−Paleoproterozoic uplifted footwalls of the Xiaoqinling metamorphic core complex (XMCC), is one of China’s largest gold producers; however, achieving a consensus regarding their metallogenic model remains elusive. Scheelite is an indicator mineral that commonly occurs in lode gold deposits worldwide used to recognize deposit types and understand hydrothermal evolution and the origin of features. Xenotime, monazite, and rutile are common hydrothermal minerals in association with lode gold deposits worldwide. Here, we provide textual, in situ U-Pb geochronology of xenotime, monazite, and rutile, and in situ elemental and Sr-Nd isotopic compositions of scheelite within different stages from the large Yangzhaiyu lode gold deposit, aiming to elucidate its genesis and, for the first time, establish a holistic correlation between the lode gold mineralization and the evolution of the XMCC. Notably low εNd(t) values (−30.7 to −23.7), high 87Sr/86Sr ratios (0.72659−0.75914), and distinct rare earth elements, Sr, Mo, and As contents of scheelite confirm a metamorphic crustal source. Xenotime U-Pb dating and pre-ore (Stage I) scheelite reveal that ore-barren metamorphic fluids at ca. 140 Ma were oxidized with low Bi contents and buffered by greenschist facies metamorphism when the XMCC initiated. Monazite and rutile U-Pb dating combined with ore-stage scheelite geochemistry indicate a compositional shift in the more reduced auriferous metamorphic fluids, which dominated during major gold deposition periods (stages II and III) from 130 Ma to 120 Ma, characterized by significantly depleted Na and increased Bi contents. This resulted from the prograde greenschist-to-amphibolite metamorphism at mid-lower crustal depths as the result of the XMCC isostatic doming and the lithospheric mantle thinning after 130 Ma. This study highlights the crucial role of metamorphic core complexes in governing the timing, locations, and resources of the lode gold metallogenic system.

The Zaozigou orogenic gold-antimony deposit, West Qinling Orogen, China: Structural controls on multiple mineralization events

Orogenic gold deposits are generally thought to represent one perhaps protracted event. However, recent research on orogenic gold deposits increasingly offers evidence for some deposits forming through multiple and clearly discreet hydrothermal episodes. The giant Zaozigou orogenic Au-Sb deposit in the Triassic to Cretaceous West Qinling Orogen, central China, includes both steeply dipping and gently dipping orebodies. The two distinct mineralization styles provide a valuable setting for investigating a multiple mineralization model by integrating structural analysis within a robust geochronological framework. Through fieldwork and geochronology, we define a progression of major tectonic events in the area of the Zaozigou deposit. The deposit is hosted within a well-bedded sequence of Early Triassic metasedimentary rocks of the South Qinling Terrane. Pre-mineralization E-W shortening (D1) during subduction of the Mianlue oceanic slab include folding with resulting axial planes striking N-S, emplacement of Triassic ENE-striking and WNW-striking dacite dikes accompanied by Middle Triassic greenschist facies metamorphism. Late Triassic gold-stibnite quartz vein and disseminated mineralization formed along ENE-striking and steeply dipping D2 brittle to ductile sinistral faults. Their orientations suggest a link to the regional NNE-SSW maximum principal stress coinciding with transpression caused by the Late Triassic collision between the South China Block and South Qinling Terrane. Overprinting Early Cretaceous quartz-stibnite veins developed along gently dipping (20° to 40°) brittle D3 normal fault zones, which exhibit a NE-SW minimum principal stress. This younger deformation event is interpreted to be related to the Early Cretaceous tectonic transition from shortening to extension of the West Qinling Orogen. Therefore, the Zaozigou deposit reveals a model of multiple orogenic gold mineralizing events, with migration of hydrothermal fluids during discrete deformation episodes and the resulting formation of a single composite deposit formed along overprinting structures at separate times of orogenesis.

Final suture of the Palaeo-Asian Ocean domain: Insights from the geochemistry and geochronology of the early to Middle Permian Elitu Formation volcanics

The consumption of the Palaeo-Asian Ocean domain triggered substantial resources and environmental effects, but the final suture timing remains debated. This study presents the zircon U-Pb age and Hf-O isotope, whole–rock element and Sr–Nd–Pb isotope data from the Elitu Formation trachyandesite and rhyolitic tuff in the Bainaimiao arc belt. The zircon U-Pb ages (260–280 Ma) indicate that the Elitu Formation volcanism erupted at the Kungurian to Capitanian. Trachyandesites exhibit an arc-like whole–rock trace element composition and have enriched Sr–Nd–Pb isotopic composition (ISr(t) = 0.7062–0.7065; εNd(t) = −8.8 to −9.4; (206Pb/204Pb)t = 17.14–17.31) and high zircon δ18O (6.08‰–7.59‰) values, which are derived from the partial melting of the lithosphere mantle metasomatised by ancient sediment-derived melts. Rhyolitic tuffs have variable εHf(t) (−7.15 to +10.47) and low δ18O (4.99‰–6.21‰) values and exhibit affinities with I-type felsic rocks. They mainly originate from the partial melting of the basement of the Bainaimiao arc belt. Our compiled geochemical and geochronological data indicate that there were significant differences between the Early to Middle Permian Elitu Formation volcanic rocks and intrusive counterparts and the Early to Middle Triassic magmatic rocks from the Bainaimiao arc belt, and the Late Permian was the key tectonic transition period. The Late Permian transition was compatible with the regional crustal-shortening deformation, greenschist facies metamorphism and shift of deposition environment. This study argues that the Elitu Formation volcanism should be subduction-related and predate the final suture of the Palaeo-Asian Ocean domain at the Changhsingian. The suture of the Palaeo-Asian Ocean domain was not the original trigger of the Kungurian climate change in the North China Craton.

Metamorphic and deformational characteristics recorded by Langzishan Barrovian series rocks within the Jiao-Liao-Ji Orogenic Belt, North China Craton

Orogenesis always involves multiple tectonic events accompanied by metamorphic cycles. The Barrovian metamorphic series often records complicated prograde metamorphism and deformational features, which are crucial for understanding the related orogenic evolution. While Barrovian metamorphism have been widely documented in the Phanerozoic orogenic belts, limited stuides have been conducted on Paleoproterozoic Barrovian metamorphic series. In this contribution, Barrovian metamorphic series extending from biotite to kyanite zones in the middle of Jiao-Liao-Ji Orogenic Belt (JLJOB) from eastern part of North China Craton (NCC) was investigated. The current studies identified the occurrence of polyphase deformation and metamorphism within the Langzishan Barrovian metamorphic series. D1 fabrics are characterized by the development of S1 foliation and L1 lineation in association with greenschist facies metamorphism (M1) that occurrs at ca. 490 ℃ and ca. 5.7 kbar. D2 fabrics that include different folds (F2), S2 crenulation cleavage, S2 foliation and L2 lineation, accompanied by the low amphibolite metamorphism (M2) that occurrs at ca. 670 ℃ and 8.2–8.6 kbar. D3 fabrics involving kink band postdate sillimanite growth that occurrs at ca. 660 ℃ and ca. 6.5 kbar, and predate chlorite occurrence. Monazite U-Pb dating suggests the peak and retrograde metamorphic ages of ca. 1.93 Ga and 1.86 Ga, respectively. Thus, a clockwise pressure–temperature-time-deformation (P-T-t-D) path is established, possibly related to the crustal thickening process. These features are very similar to those documented in Phanerozoic Barrovian metamorphic series, suggesting that a comparable orogenesis mechanism could be shared in the Paleoproterozoic and Phanerozoic.

PT conditions of deformation of the Gol-e-Gohar shear Zone, SW Iran: Insights from analysis of quartz c-axis fabrics, recrystallization mechanisms and syndeformational fluid inclusions

An investigation into the correlation between microstructural characteristics and deformation temperatures of quartz in the middle crust is of utmost importance in yielding substantial knowledge of the nature of deformation zones. Mylonitic rocks from the Gol-e-Gohar Shear Zone were investigated to infer innovative insights into the ductile deformation pressure–temperature (PT) conditions of the Sanandaj–Sirjan metamorphic rocks within the Zagros, southwest Iran. This approach used a multi-method strategy including quartz c-axis fabrics, recrystallization mechanisms and fluid inclusions (Fls) data. Geothermobarometric analysis based on these data highlight deformational PT conditions ranging from ∼400 to ∼500 °C and P < 5 kbar that correspond to ductile deformation with greenschist to lower-amphibolite conditions at depth of ∼ 12–19 km. Measured opening angles of quartz c-axis fabric and dominance of basal < a > slip system indicate crystal plastic deformation occurred in T > 400 °C. Microstructures highlighting the occurrence of Grain Boundary Migration (GBM) confirm temperature of 500–600 °C. The deduced P–T–salinity Fls history of deformation-related fluid inclusions is highlighted by Fls (B-type) with H2O-CO2–rich metamorphic fluids. These fluids were trapped during ductile deformation in temperatures ranging from ∼ 355 to ∼ 535 °C and pressure of P < 5 kbar. The petrofabric analyses show that the mylonitic rock in the Gol-e-Gohar shear zone have undergone metamorphism at the amphibolite facies condition, and then followed by a retrograde greenschist facies metamorphism. These results emphasize the utility of microstructural and fluid inclusion techniques to constrain the history of mylonitic rocks exhumation in the deformation zones.

Microfabrics, In Situ Trace Element Compositions of Pyrite, and the Sulfur Isotope Chemistry of Sulfides from the Xitieshan Pb-Zn Deposit, Qinghai Province, Northwest China: Analysis and Implications

The Xitieshan deposit, located in the central segment of the northern margin of the Qaidam Basin, is among the largest massive Pb-Zn sulfide deposits in China. This deposit, along with its ore-bearing rock series known as the Tanjianshan Group, underwent greenschist facies metamorphism due to subsequent orogeny. We investigated the in situ sulfur isotopes of sulfides with different occurrences to define the origin of ore-forming fluids. The δ34S values of sulfides from stratiform ores, massive ores in schist, stockwork ores in marble, schist and discolored altered rocks that constitute a typical double-mineralization structure range from −5.3‰ to +5.6‰ and from −1.7‰ to +32‰, respectively, indicating distinct biological and thermochemical reductions in seawater sulfates. These are similar to the sulfur isotopic characteristics of VSHMS deposits. Pyrite, whose LA-ICP-MS trace element compositions can provide significant information about metallogenic evolution and deposit genesis, is ubiquitous throughout the whole mineralization process. In these stratiform, massive and stockwork ores, three pyrite types were identified: colloform pyrite (Py0), fine-grained anhedral spongy pyrite (Py1) and coarse-grained euhedral pyrite (Py2). The contents of most metallogenic elements, such as Cu, Pb, Zn, Ag, Mo, Mn and Sn, decrease from Py0 to Py2 with the enhancement of metamorphic recrystallization. This suggests that the expelled elements appear as inclusions in primitive pyrite, contributing to the precipitation of new sulfide phases, such as sphalerite and galena. Orogenic metamorphism played an important role in controlling further Pb-Zn enrichment of the Xitieshan deposit. Moreover, there is another mineralization type, primarily occurring as sulfide veins in the undeformed Formation C siltstones of the Tanjianshan Group, which also crosscut early-formed sulfides, showing close-to-zero S isotopic compositions. In this mineralization type, pyrite (Py3) displays high Se/Tl (&gt;10) and Co/Ni (&gt;2.2) ratios, both indicating a minor superimposed post-orogenic magmatic–hydrothermal event.

Carbonate clumped isotopes and blocking temperatures of marbles from the Backbone Range, Taiwan

Clumped isotopic compositions (abundance of 13C16O18O in CO2, expressed by Δ47) in massive calcite marble (CM) and dolomite marble (DM) were reported to refine our understanding of greenschist-facies metamorphism during the still on-going arc-continent collision from the northern, central, and southern localities along the northern-central Tailuko belt of the Backbone Range of Taiwan. Black calcite marble (BCM), white calcite marble (WCM), and DM lenses with cataclastic texture were analyzed. We observed that the carbonate clumped isotope reordering is independent of the grain size of minerals and is less likely affected by lattice defects and active stress. The averaged Δ47 values for CMs and DMs are 0.308 ± 0.004 ‰ (n = 26) and 0.235 ± 0.003 ‰ (n = 13), respectively, giving blocking temperatures of 211 ± 6 °C and 332 ± 8 °C. The blocking temperatures infer mean cooling rates of 614 °C/m.y. and 8.8 × 104 °C/m.y for calcites and dolomites, respectively, estimated based on the available models and clumped reordering mechanisms. We noted that the variations in cooling rates widely differ with different reordering models, especially with different temperature calibrations. Some cooling rate estimates are in agreement with those based on zircon/apatite thermochronological studies. Further refinements on Δ47 - temperature calibration, reordering mechanism and estimation of thermodynamic constants, in addition to improvement in analytical precision, are needed for practical applications of carbonate clumped thermometry as a tectonic speedometer for metamorphic terranes.

Contractional deformation between extensional dome exhumation in Central Pamir at 17–15 Ma constrained by metamorphic and paleomagnetic data from the Bartang volcanic rocks, Tajikistan

The debated mechanism and timing of formation of the Pamir orogenic salient provides an ideal case to combine paleomagnetic and metamorphic analyses. Opposing views argue for either oroclinal bending of the Pamir since the India-Asia collision or for an originally arcuate shape, which can be tested using paleomagnetism to estimate vertical-axis rotations. Furthermore, Pamir deformation can be dated and characterized by the analysis of a well-expressed regional metamorphism. However, paleomagnetism is seldom applied to metamorphic rocks such that the significance of their rock magnetic signal with respect to deformation remains poorly understood. We studied a > 2 km thick sequence of slightly metamorphosed Cretaceous-Paleogene volcanic and volcaniclastic rocks from the western Central Pamir Mountains using metamorphic petrology, geochronology, and paleomagnetic analyses. These rocks present a medium-grade metamorphism and have undergone fine grained recrystallisation of biotite due to NW-SE compression. 40Ar/39Ar and Rb/Sr ages (∼17 to 15 Ma) on whole rock and biotite, confirm that the greenschist facies metamorphism is related to the coeval exhumation of Pamir gneiss domes during the Middle Miocene. The rocks generally show homogenous magnetic properties. High field magnetic properties and SEM/EDS data show that the main magnetic remanent carrier is titano-hematite. Thermal demagnetizations yield complex characteristic remanent magnetization with site-mean directions obtained for only 14 of the 44 studied sites but scattered mainly within the foliation plane defined by the anisotropy of magnetic susceptibility (AMS). These rocks have also unusual magnetic characteristics where strong-field isothermal remanent magnetizations induce an AMS fabric, likely related to the specific composition and structure of the metamorphic titano-hematite but independent of the original anisotropy fabrics. Combined paleomagnetic and geochronological data from the Bartang volcaniclastics rocks enable us to identify a ∼ 17–15 Ma compressive deformation event contemporaneous with the emplacement of the gneiss domes and suggest no oroclinal bending since that event.

Evidence for a pre-Himalayan metamorphism in the High Himalayan Crystalline of the Miyar Valley (NW India)

Most of the tectonic, metamorphic and geochronological data suggest that the Himalaya is essentially the consequence of a single orogenic cycle associated with the India-Asia collision during the Cenozoic era. Therefore, metamorphic assemblages and tectonic structures across the Himalayan range are systematically considered as post-collisional geological records. However, over the last decades, several observations arguing for geological events predating the continental collision have become increasingly recurrent in the literature. Nevertheless, although some of these arguments are thoroughly documented, they are unduly ignored in the construction of models drawing the tectono-metamorphic evolution of the Himalayan range. Yet, the occurrence of a pre-Himalayan history would have considerable consequences on the classical models for the building of the Himalaya. The recent discovery of inclusions of staurolite crystals in greenschist facies garnets from the Miyar Valley in Upper Lahul region (Himachal Pradesh; NW India) revives the debate on the existence of a pre-Himalayan metamorphism. Indeed, the occurrence of high-temperature staurolites included in greenschist facies garnets suggests that the High Himalayan Crystalline rocks experienced an amphibolite facies metamorphism prior the predominant Himalayan greenschist facies metamorphism observed in this part of the range. In this study, phase petrology, microtectonic investigations combined with preexisting geochronological data infer that the crystallization of the included staurolite predates the growth of Himalayan garnets. These original data bring new arguments to bear on the long lasting debate of the existence of a Pre-Himalayan orogenic cycle. They lead to the conclusion that the growth of staurolite predates the continental collision between India and Asia and reflects a metamorphic event that belongs to a pre-Himalaya orogenic cycle.

Mineralogy and mineral chemistry of the ABM replacement-style volcanogenic massive sulfide deposit, Finlayson Lake district, Yukon, Canada

The ABM deposit is a bimodal-felsic, replacement-style volcanogenic massive sulfide deposit (VMS) that is hosted by back-arc affinity rocks of the Yukon–Tanana terrane in the Finlayson Lake VMS district, Yukon, Canada. Massive sulfide zones occur as stacked and stratabound lenses subparallel to the volcanic stratigraphy, surrounded by pervasive white mica and/or chlorite alteration. Remnant clasts of volcanic rocks and preserved bedding occur locally within the massive sulfide lenses and indicate that mineralization formed through subseafloor replacement of pre-existing strata. Three mineral assemblages occur at the ABM deposit: (1) a pyrite–chalcopyrite–magnetite–pyrrhotite assemblage that is associated with Cu–Bi–Se–Co-enrichment and occurs at the center of the massive sulfide lenses; (2) a pyrite–sphalerite assemblage, which occurs more commonly towards lens margins and is enriched in Zn–Pb–Ag–Au–Hg–As–Sb–Ba; and (3) a minor assemblage comprising chalcopyrite–pyrrhotite–pyrite stringers associated with pervasive chlorite alteration, which occurs mostly at the sulfide lens margins. Petrographic observations of preserved primary, zone refining, and metamorphic textures in combination with in situ geochemistry show that the pyrite–sphalerite assemblage formed at lower temperatures (< 270 °C) than the other two mineral assemblages (~ 270–350 °C), and that mineral chemistry in all mineral assemblages was affected by greenschist facies metamorphism, although the effects are limited to recrystallization, small-scale remobilization (< 1 m) and trace element redistribution.

Characterization of the Angat Ophiolite: New insights from bulk major and trace element geochemistry and petrographic analysis

The Angat Ophiolitic Complex, located north-northeast of Manila, Luzon is perhaps one of Luzon’s most fundamental suspect terranes. The definite age of the ophiolite is not well-established, with various authors having different claims. Encarnacion et al. (1993) conducted a U-Pb age dating for the Angat wherein the Angat Ophiolite was found to have an Early Middle Eocene age (48 Ma), close to the ZOC age - leading to the assumption that the two ophiolites are related and underlie most of Central Luzon. Hence, this paper aims to collect samples from the Eocene and Cretaceous Angat Ophiolite for petrographic and geochemical analyses, identify the relationship between the separated basalt units from the basalts in the main ophiolite body, and compare the geochemistry of the Eocene and Cretaceous units of the Angat Ophiolite with the other well-known ophiolites from the literature. Field investigations were conducted in five important localities wherein 15 samples were obtained. The results highlighted that the major mineral phases in the samples collected are plagioclase and augite, with some samples having hornblende and olivine. All samples appear to have undergone low-grade greenschist facies metamorphism, which may be attributed to hydrothermal alteration. Chloritization of pyroxene minerals is also evident, along with the hydrothermal alteration products of plagioclase and pyroxenes. The relationship between the EAO, MOC, and separated basalt patch in Marilaque Highway is identified. The Coto Block of the Zambales Ophiolite and the Angat Ophiolite may be related and formed over a mantle initially enriched by a subduction component.

Paleoproterozoic rift-related volcanism of the Karrat Group, west Greenland: New insights from lithofacies analysis and geochemistry of the Kangilleq Formation

Mafic volcanic rocks of the Paleoproterozoic Kangilleq Formation of the Karrat Group of West Greenland represent a submarine mafic volcanic sequence, erupted and deposited on the Rae craton of Greenland. The formation ranges from <100 m-thick to ∼500 m-thick, and consists of mafic volcaniclastic rocks, pillow lavas and massive basalt lavas that have undergone greenschist facies metamorphism, yet preserve primary volcanic textures such as pillows, drainage cavities, lobes, flow banding, autoclastic breccias, hyaloclastite, scoria and layering.Rocks of the Kangilleq Formation overlie the Archean Qeqertarssuaq Complex and are interleaved with and overlain by the ca. 1950 Ma Nûkavsak Formation. Based on detailed (1:200) mapping and high-precision geochemistry, rocks of the Kangilleq Formation are subdivided into the Kangiusiap and Umiammakku members. The Kangiusiap member is dominated by mafic, scoria-rich breccias, has sulfide mineralization, and is bound at the bottom and top by the Nûkavsak Formation. This member has subalkaline, transitional to tholeiitic, mid-ocean ridge basalt (MORB) to back arc basin basalt (BABB) signatures with a Nd-isotope value similar to the depleted mantle at 1.88 Ga. The Umiammakku member is dominated by pillow lavas, with subordinate amounts of massive lavas and volcaniclastic rocks, overlies the Qeqertarssuaq Complex and is overlain by, and interleaved with, the Nûkavsak Formation. It is alkaline with characteristics similar to modern ocean island basalts and Nd-isotopic values slightly lower than the depleted mantle at 1.88 Ga.The rocks of the Kangilleq Formation represent non-arc suites or suites that have only minor subduction zone or crustal input, and display geochemical signatures similar to basalts found in modern intracontinental back arc settings. Lithofacies, bedforms, internal stratigraphy, and the identification of synvolcanic faults, suggest it is the product of volcanic eruptions during incipient rifting within a back arc setting, and seafloor mass wasting events at fault scarps related to rifting. Based on the geological characteristics and its petrogenetic and tectonic history, the Kangilleq Formation is interpreted to be correlative with the Bravo Lake Formation of the Piling Group of northeastern Canada and records incipient rifting of the southern margin of the Rae craton in the Paleoproterozoic.

Monazite–xenotime thermometry: a review of best practices and an example from the Caledonides of northern Scotland

Abstract Monazite–xenotime thermometry is a potentially powerful technique for understanding the evolution of Earth systems. While a rich set of experimental and empirical datasets are available for monazite–xenotime equilibria, five different thermometric calibrations yield significantly different results, making this technique difficult to apply in practice. To clarify best practices for monazite–xenotime thermometry, a compilation of published compositional data for monazite and xenotime with independently determined pressure–temperature conditions is evaluated. For each existing thermometer, we examine how closely estimated temperatures match independent empirical temperatures and consider how best to calculate monazite end-members for each thermometer. Monazite–xenotime thermometry is applied to samples from the Northern Highlands Terrane of northern Scotland, which experienced amphibolite–upper greenschist facies metamorphism and penetrative deformation during the Scandian orogeny. Thermometry data in conjunction with U–Pb dating define relatively slow regional cooling across the Scandian thrust nappes. Thermometry data closely match quartz c -axis fabric-based deformation thermometry across the structurally lower nappes, suggesting that monazite and xenotime record the timing and temperature of penetrative deformation and shearing. The data suggest that ductile deformation in the hinterland nappes of the Scandian orogen in Scotland occurred as late as 415–410 Ma.

Monazite and xenotime petrochronologic constraints on four Proterozoic tectonic episodes and ca. 1705 Ma age of the Uncompahgre Formation, southwestern Colorado, USA

Abstract The Proterozoic tectonic evolution of the southwestern USA remains incompletely understood due to limited constraints on the timing and conditions of the tectono-metamorphic phases and depositional age of metasedimentary successions. We integrated multi-scale compositional mapping, petrologic modeling, and in situ geochronology to constrain pressure-temperature-time paths from samples of Paleoproterozoic basement gneisses and overlying quartzites in southwestern Colorado, USA. Basement gneiss from the western Needle Mountains records metamorphic conditions of 600 °C at 0.75 GPa at 1764 ± 9 Ma and ~575 °C at 1741 ± 10 Ma. Gneiss sampled from drill core near Pagosa Springs, Colorado, records conditions of 700 °C at 1748 ± 9 Ma, 800 °C at 1.1 GPa at 1650 ± 40 Ma, 540 °C at 1570 ± 36 Ma, and 440 °C at 1424 ± 12 Ma. The Uncompahgre Formation was deposited at ca. 1705 Ma, as constrained by detrital monazite (1707 ± 8 Ma) and xenotime (1692 ± 40, 1725 ± 50 Ma), metamorphic xenotime (1650 ± 10 Ma), and published 40Ar/39Ar and detrital zircon data. Compositions of ca. 1705 Ma detrital monazite and xenotime are consistent with derivation from a garnet-bearing source in the Yavapai orogenic hinterland. The Vallecito Conglomerate and Uncompahgre Formation record macroscopic folding and greenschist-facies metamorphism at 1650 ± 10 Ma and temperatures of 270 °C to &amp;gt;570 °C at 1470–1400 Ma. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) zircon geochronology yielded dates of 1775 ± 18 Ma from the Twilight Gneiss and 1696 ± 7 Ma from the Bakers Bridge Granite, supporting previous isotope dilution–thermal ionization mass spectrometry (ID-TIMS) dates. The Eolus Granite yielded a date of 1463 ± 6 Ma, which is older than previous 1.44–1.43 Ga ID-TIMS dates. The newly dated granite of Cataract Gulch is 1421 ± 12 Ma. In situ analysis of detrital and meta-morphic monazite and xenotime, igneous zircon, and quantitative thermobarometry, integrated with previously published constraints, indicate multiple tectonic episodes after the emplacement of 1800–1760 Ma arc-related rocks. The region experienced greenschist- to amphibolite-facies metamorphism (M1) from 1760 Ma to 1740 Ma, which was followed by the intrusion of granites at 1730–1695 Ma and deposition of the Uncompahgre Formation at ca. 1705 Ma, contemporaneous with the Yavapai orogeny. Metamorphism at 1680–1600 Ma was characterized by greenschist-facies conditions near Ouray, Colorado, and granulite-facies conditions near Pagosa Springs (M2) during the Mazatzal orogeny. From 1470 Ma to 1400 Ma, greenschist- to amphibolite-facies metamorphism (M3) and largely granitic plutonism occurred during the protracted Picuris orogeny. These results demonstrate the power of monazite and xenotime analyses to constrain depositional ages, provenance, and pressure-temperature-time (P-T-t) paths to resolve the compound orogenic history that is characteristic of most mountain belts.

A craton-wide geochemical study of Neoarchaean carbonate rocks of Zimbabwe

Neoarchaean greenstone belts of the Zimbabwe craton host microbialite-bearing limestone successions that range in age from ~2.82 to 2.68 Ga. The best-preserved successions, according to Raman geothermometry, are situated in the Bulawayo and Belingwe greenstone belts where they have been subjected to lower greenschist facies metamorphism with a peak temperature of ~350 °C. Elsewhere, such as in the Masvingo belt, peak temperatures reached ~540 °C, giving rise to marbles. The carbonate rocks consist mainly of calcite and, to a lesser extent, dolomite and contain variably admixtures of siliciclastic detritus. Samples with the least amount of siliciclastic detritus have shale-normalized rare earth element and Y distribution patterns that indicate open marine conditions, with positive La, Eu, and Gd anomalies, superchondritic Y/Ho ratios, and depleted light rare earth elements relative to the heavy rare earth elements. The δ13СVPDB values are −2.0 to 2.1 ‰, and δ18OVPDB values cluster in the range of −20 to −10 ‰. Strontium isotope ratios vary in concert with siliciclastic detritus. The lowest initial 87Sr/86Sr ratio of 0.70155 is close to the depleted mantle value at the time of deposition, suggesting minimal contribution from evolved felsic crust to the Neoarchean ocean. Only samples from the Masvingo belt show radiogenic Sr isotope ratios (0.70202–0.70340) indicative of an epicratonic setting with exposed felsic basement. For the rest, open marine conditions prevailed in a proto-cratonic setting characterized by mantle-derived greenstone volcanism. Carbonate sedimentation on the Zimbabwe craton thus predate the fundamental rise in the marine 87Sr/86Sr Sr isotope signal in the late Neoarchaean.

Geochemical and remote sensing integrated with satellite gravity data of Darhib and Atshan talc deposits, South Eastern Desert, Egypt

The current contribution conducted new geochemical, remote sensing integrated with gravity detailed studies of talc deposits to identify the talc protolith as well as its extension, depth, and structures. There are two examined areas, distributed from north to south, Atshan and Darhib and both belong to the southern sector of the Egyptian Eastern Desert. They occur as individual lenses or pocket bodies in ultramafic-metavolcanics following NNW-SSE and E-W shear zones. Geochemically, among the investigated talc, Atshan samples have high contents of SiO2 (av. 60.73 wt.%), and higher concentrations of transition elements such as Co (av. 53.92 ppm), Cr (781 ppm), Ni (av. 1303.6 ppm), V (av. 16.67 ppm), and Zn (av. 55.7 ppm). Notably, the examined talc deposits contain low contents of CaO (av. 0.32 wt.%), TiO2 (av. 0.04 wt.%), SiO2/MgO (av. 2.15), and Al2O3 (av. 0.72 wt.%), which is comparable with ophiolitic peridotite and forearc setting. False color composite (FCC), principal component analysis (PCA), minimum noise fraction (MNF), and band ratio (BR) have been used to distinguish talc deposits in the investigated areas. Two new proposed band ratios were created to separate talc deposits. FCC band ratios (2/4, 4/7, 6/5) and (4 + 3/5, 5/7, 2 + 1/3) have been derived to focus on talc deposits in two case studies, Atshan and Darhib areas. The application of regional, residual, horizontal gradient (HG), and analytical signal (AS) techniques to gravity data are used in interpreting the structural directions of the study area. The analysis of this technique displays several notable faults trending in NW–SE, NE–SW, NNW–SSE, and E–W directions. Two techniques of gravity depth calculation were applied in the study areas, namely source parameter image (SPI), and Euler deconvolution (EU). The analysis of these techniques reflects that the depth of subsurface sources ranges between 383 and 3560 m. Talc deposits may be attributed to greenschist facies metamorphism or to a magmatic solution that is (associated with granitic intrusions) interacted with the surrounding volcanic rocks forming metasomatic minerals.

Petrogenesis of high-Mg andesites from the Proterozoic Shillong Basin, Northeast India: evidence for continuation of the Central Indian Tectonic Zone to the Assam-Meghalaya Gneissic Complex and its implications for the Columbia supercontinent reconstruction

ABSTRACT This study provides comprehensive geochemical and petrological data for the newly discovered high-magnesium andesites (HMAs) from the Shillong Group of rocks in the Proterozoic Shillong Basin of the Assam-Meghalaya Gneissic Complex (AMGC). It is the first report of HMAs from the easternmost part of the Indian shield region. The AMGC holds immense geodynamic significance because this region has recorded magmatism related to Columbia, Rodinia, and Pangea supercontinental events. The present work discusses geochemical signatures of the Shillong HMAs characterized by calc-alkaline traits with high-K contents and significant concentrations of TiO₂ (0.54–0.76 wt %) higher than boninites (TiO2 < 0.5 wt %), and Mg# (53–56). They exhibit high LILE/HFSE and LREE/HREE ratios, which are distinctive features of magmas generated in subduction zones. Tectonic discrimination diagrams clearly suggest that the Shillong HMAs were generated in a back-arc basin regime. We argue that the Shillong HMAs were derived by 5% to 10% partial melting of a depleted lherzolite mantle source within the spinel – garnet transition zone. We also argue that the Shillong HMAs may have formed as a result of the interaction between subducted sediment-derived melts and mantle. The Shillong HMAs have undergone low- to medium-grade greenschist facies metamorphism characterized by significant mineral alteration, however, they still preserve bulk original alteration-resistant trace element compositions. From a regional perspective, the synchronous nature of origin and geotectonic setting, associated geochronological ages of formation, and regional structural trends suggest that the Shillong Basin is the easternmost continuation of the Mahakoshal Mobile Belt and specifically, the Bathani volcano-sedimentary sequence of the Central Indian Tectonic Zone (CITZ); and together they form the northern fragments of the same suture zone consisting of the CITZ, the Chotanagpur Granite Gneiss Complex, and the AMGC, which demarcates the northern and southern Indian blocks of the Greater Indian Landmass.

A nearly isobaric P-T-deformation evolution path of the Cerro Negro Paleoproterozoic mylonitic igneous suites: The exhumation of the Tandilia Belt of the Rio de la Plata Craton

Two high-grade mylonites from Cerro Negro hill were studied in detail to shed light on the igneous-metamorphic evolution of the northwestern extreme of the Azul Megashear Zone (AMSZ) of the Tandilia Belt, Rio de la Plata craton. Bulk-rock compositions of a garnet-bearing hornblende gneissic mylonite (ribbon mylonite) and a xenolith of garnet-bearing granite mylonite (striped gneiss) included in the former were modeled with the GeoPs software (Xiang and Connolly, 2021) to construct P-T pseudosections in the 11-component system Si–Ti–Al–Fe–Mn–Mg–Ca–Na–K–O–H by Gibbs energy minimization. Pseudosections were contoured for isopleths parameters and mineral isomodes of interest. Garnet, with a homogeneous composition of pyr5(gro + andr)18.5spes6.5alm70, present in the younger granite crystal mush (<8 vol% of melt for 0.6 wt% of H2O) formed at 820 °C and 7.2 kbar. These conditions were encompassed by biotite with 0.21–0.23 Ti pfu and amphibole with Mg# of 0.30–0.33. A nearly isobaric cooling is outlined during a high-grade deformational event (D1) until core with the composition of pyr5(gro + andr)10spes5alm80, in slightly zoned garnet of the granite xenolith (modeled with 0.3 wt% of H2O), yields conditions of 540–580 °C and 7.2–8.2 kbars. Rimwards, a subsequent cooling with exhumation was recorded in garnet composition of pyr3(gro + andr)17spes6alm72 to reach conditions of ∼420–440 °C and ∼6.6 kbar. The integrated P-T path holds conditions of middle crust realms (ca. 24–25 km) from the initiation of deformation (D1) coevally to, or shortly, after the younger granite crystallization with a waning apparent geothermal gradient to 20 C°Km−1. Consistent mechanisms of deformation in minerals (d) generated a subvertical E-W mylonitic foliation (D1) under granulite to amphibolite metamorphic facies conditions until greenschist metamorphic facies conditions (D2). Fragile events (D3 and D4) occurred at higher crustal levels concomitant with water influx and sporadic retrogression. Hitherto, the studied rocks experienced a somewhat different evolution path when compared to adjacent Paleoproterozoic areas around the AMSZ or from other places of the Tandilia Belt, which show gently partial exhumation P-T paths after their solidus.

Upper plate exhumation during continental collision: An example from the paranapanema paleocontinental active margin, SE Brazil

The Southern Brasília Orogen is the result of Ediacaran continental collision between the active margin of the Paranapanema paleocontinent and the passive margin of the São Francisco paleocontinent. The active margin was deformed into the Socorro-Guaxupé Nappe and the associated Embu Domain. This paper presents new U–Pb zircon data and geothermobarometric estimates that allow to reconstruct the metamorphic evolution in these upper plate units. A pre-collisional metamorphism M1 with an age peak in metamorphic rims of zircon grains in the range 680–630 Ma is related to the development of the Socorro-Guaxupé magmatic arc with batholiths dated in a similar time span, 690 - 630 Ma. This metamorphism reached medium pressure granulite facies, now exposed by the tilted uplift and erosion due to the collision. Another peak is detected in metamorphic rims of zircon grains, in the range 630–590 Ma, labeled M2, related to the collision and corresponds to the main metamorphism in the lower plate and accretionary wedge-like nappe stack, underlying the Socorro-Guaxupé Nappe. This nappe stack was affected by greenschist facies metamorphism in the autochthonous underlying craton, grading to high pressure granulite facies in the upper nappe, with local occurrences of retroeclogite lenses. In the Socorro-Guaxupé Nappe and Embu Domain this main metamorphism (M2) is characterized by three metamorphic zones decreasing in grade with distance from the suture: a granulite facies orthopyroxene bearing zone; a high temperature amphibolite facies zone and a lower amphibolite facies zone. Geothermobarometric results in mineral associations related to M2 show temperatures decreasing from about 770 °C to ∼620 °C and pressures decreasing from about 9 kbar close to the suture to about 5 kbar in the southern and western parts of the studied region. On the upper Paranapanema plate, relative to the Socorro-Guaxupé Nappe and Embu Domain units, at the base of the plate, an older metamorphism (M1) is verified with temperatures between 960 and 890 °C and pressures between 11 and 9 kbar, reequilibrated at approximately 700 °C with a pressure of about 9 kbar (M2, continental collision). For this reason, it can be assumed that the rim of the Paranapanema plate was exhumed during the continental collision, since the temperature and pressure of the metamorphic peak determined during the continental collision are lower than those determined for the pre-collisional magmatic arc metamorphism.