Crustal Terranes Research Articles

Geological interpretation of wide-angle seismic reflection/refraction profiles along the Scotian margin and across Nova Scotia, Canada

Two wide-angle seismic reflection/refraction profiles were acquired to determine the velocity characteristics of the Meguma terrane and adjacent Avalonia northeast of the Cobequid-Chedabucto Fault Zone (CCFZ) in eastern Canada. Line 99–1 is located along the Scotian margin whereas line 99–2 crosses the Scotian margin and onshore Nova Scotia and extends into the Gulf of St. Lawrence. Velocity models were derived by forward modelling of travel times. P-wave velocities of 5.5–6.0 km/s and 6.0–6.4 km/s are interpreted to be metasedimentary rocks of the Goldenville and Halifax groups and granitic rocks, respectively. This inference is based on comparison with laboratory velocities of a suite of rock samples, offshore sampling, links to onshore geology with multibeam bathymetry, and gravity modelling. On both lines a low velocity zone (LVZ) >350 km wide and up to 15 km thick is present at mid-crustal levels. The LVZ is not observed beneath onshore Nova Scotia or northeast of Orpheus Graben, the offshore extension of the CCFZ. The crustal thickness of the Meguma terrane varies between 31 and 38 km; in contrast, the Moho is at a depth of 41 km beneath Avalonia. The velocity of the lower crust on line 99–2 is 6.8–7.1 km/s beneath the LVZ and Avalonia, and 6.6 km/s beneath onshore Meguma terrane. Poisson's ratios calculated from P- and S-wave velocities are 0.19–0.23 in the Meguma terrane crust; in contrast, Poisson's ratios of 0.24–0.25 are indicated for Avalonian crust.

Seismic probing of buried ancient terrane boundaries - insight into Fennoscandia’s Palaeoproterozoic continental formation

Terrane boundaries are usually mapped from structures, lithologies and ages of the bedrock geology. Geophysical studies occasionally used to image terrane boundaries at depth typically yield localised information. In this contribution we demonstrate that it is possible to map Palaeoproterozoic terrane boundaries over large areas in northwest Fennoscandia through modelling of the crustal structure using receiver function inversion. Our new model suggests the existence of three “arms” of thick crust interpreted as deep expressions of Palaeoproterozoic terrane boundaries. We also observe previously unknown high velocity lower crust that we relate to Palaeoproterozoic rifting. The deep structural information shows that crustal structures can be preserved over billions of years and yields a three-dimensional crustal terrane model. This model improves our understanding of Palaeoproterozoic tectonics and continent formation processes.

Heterogeneous multi-stage accretionary orogenesis — Evidence from the Gunnison block in the Yavapai Province, southwest USA

Proterozoic rocks exposed in the southwestern U.S.A. represent one of the best examples of crustal growth by arc-related magmatism and accretionary orogenesis. Within the Southwest the 1.8–1.7 Ga Yavapai Province is widely regarded as a classic example of juvenile arc crust, however 1.8–2.5 Ga inherited zircon and Nd and Hf model ages have been recognized near Gunnison in central Colorado. These data have led to questions regarding the extent and nature of pre-1.8 Ga crustal material and the genesis of the Yavapai Province. We present evidence for a geochemically distinct, spatially restricted crustal block underlain by pre-1.8 Ga crust material (referred to here as the Gunnison block) in central to western Colorado within the Yavapai Province. The Gunnison block is characterized by 1.8–1.9 and 2.4–2.6 Ga inherited zircon, Pb isotopic systematics (μ = 9.8 ± 0.1, κ = 3.7 ± 0.1) elevated relative to 1.8 Ga depleted mantle values, 1.8–2.5 Ga Nd and Hf model ages, and a distinct pressure-temperature-time history. The geochemical data are consistent with mixing between juvenile 1.8 Ga and pre-1.8 Ga sources. The older crustal component is most similar to the isotopically enriched Mojave Province of eastern California and western Arizona, suggesting greater similarities between these provinces than previously recognized. Monazite and xenotime petrochronology indicate ca. 1.75–1.74, 1.72–1.69, 1.67, and 1.47–1.38 Ga tectono-metamorphic events. These data suggest that the Gunnison block accreted to other components of the Yavapai Province outboard of Laurentia at 1.75–1.74 Ga. The composite Yavapai Province was accreted to the margin of Laurentia during the 1.72–1.69 Ga Yavapai orogeny. Later overprinting is associated with the ∼1.68–1.60 Ga Mazatzal and ∼1.47–1.37 Ga Picuris orogenies. Identification of distinct crustal terranes within the Yavapai Province supports models involving multiple arcs and back-arcs that were progressively assembled prior to their accretion to Laurentia, perhaps akin to the present-day Banda Sea in Indonesia.

U-Pb geochronology and petrography of Neoproterozoic to early Cambrian volcanic rocks in basement crustal terranes beneath the deep-water Gulf of Mexico

Abstract Though the Gulf of Mexico has been studied for more than a century, the lithology, age, and origin of the basement crustal terranes remain poorly understood. New U-Pb zircon ages of a volcanic sample in the DeSoto Canyon 535 #1 Raptor well were obtained by in situ laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) and resulted in a crystallization age of 509 ± 23 Ma, indicating the crustal terrane beneath the northeastern Gulf of Mexico is likely of peri-Gondwanan affinity. This basement may be associated with the felsic volcanism related to Neoproterozoic to Cambrian subduction along the supercontinent’s western margin. This subduction-related volcanism likely represents the last pulses of magmatic activity before the west coast of Gondwana transitioned into a passive margin ca. 500 Ma. The data presented here for the DeSoto Canyon 535 #1 Raptor trachyte represent the oldest radiometrically dated evidence of volcanism in the present-day deep-water Gulf of Mexico.

Late Paleozoic alkaline granitoids of the southwestern and Northern Mongolia: U–Pb ID TIMS zircon dating, petrogenesis and implications for post-accretion and anorogenic activity of the Central Asian Orogenic Belt

In tectonic history of many orogenic belts, alkaline granitoids with A-type affinity mainly reveal lithosphere extension events. However, precise dating of these rocks is often hampered by several problems either due to absence of accessory zircon or due to high U and Th contents disturbing the U–Pb isotope system. This study reports results of chemical abrasion isotope dilution–thermal ionization mass spectrometry (CA ID TIMS) U–Pb dating of zircon from five plutons of aegirine- and/or alkaline amphibole-bearing alkali-feldspar granites and quartz syenites from the Trans-Altai Gobi (SW Mongolia) and Northern Mongolia in the Central Asian Orogenic Belt and reveals their close petrogenesis in different tectonic position. In the SW Mongolia the earliest Carboniferous alkaline granitoids (ca. 358–346 Ma) were formed during the post-accretion extensional event suggesting the Late Devonian accretion of the Trans-Alai Gobi–Eastern Junggar oceanic island arc to the Mongolian–Chinese Altai peri-Siberian continental margin. Alkaline granitoids of the Northern Mongolia were emplaced in the Early Permian during two independent geographically overlapping anorogenic near-coeval igneous pulses. The earliest pulse (ca. 285 Ma) related to evolution of the western half of the Northern Mongolia–Western Transbaikalia volcano-plutonic belt of alkaline granitoids with the bimodal volcanic sequences, is ascribed to the most significant Early Permian magmatic event throughout the Central Asian Orogenic Belt. The latest pluton (276 ± 1 Ma) identifies the initial emplacement stage of the Middle Permian–Early Triassic Khangai granitoid batholith.Alkaline granitoids from the Trans-Altai Gobi and Northern Mongolia possess similar ferroan, mainly peralkaline, A2-type granite geochemical characteristics, but differ in Nd isotopic signatures. For the Trans-Altai Gobi plutons εNd(t) = +6.7; for Northern Mongolian plutons εNd(t) varies between –1.7 to + 0.6. These values are consistent with the Nd isotopic evolution of the continental crust of the hosting terranes, ‘Hercynian’ isotope province for the Trans-Altai Gobi and ‘Caledonian’ province with a pre-Ediacaran microcontinent crust for the Northern Mongolia. Thereby studied alkaline granitoids were formed manly though anatexis of the host terrane crust during post-accretion and anorogenic extension events in different parts of the Central Asian Orogenic Belt.

Geological structure informs rupture propagation and surface rupture complexity during the 2016 Kaik?ura earthquake, New Zealand: insights for future large earthquake hazard

We summarise the geological setting of complex surface rupture of the 2016 Mw 7.8 Kaik?ura earthquake in the Marlborough Tectonic Domain of New Zealand. The event was complex both seismologically and geologically but not totally dissimilar to other large historical events globally. The earthquake occurred in the comprehensively imbricated, steeply-dipping Pahau Terrane crust that exhibits numerous tectonic overprints with diverse faulting styles. The current strike slip faults of the Marlborough Fault System are immature in their structural development and occupy, at least in part, inherited faults of earlier deformation phases. Several of the faults that ruptured in 2016 may connect at seismogenic depths. A listric fault geometry is likely for many of the faults that ruptured in 2016. This interpretation is supported by crustal seismic mapping identifying listric geometries for other large faults within the region. Examination of other historic surface rupturing earthquakes in the Marlborough Tectonic Domain and globally show some complexity but not to the same level of multifault rupture as in 2016. We conclude that multifault ruptures may be enhanced in the Kaik?ura region where the Australian plate crust is thinner than farther west and the plate boundary deformation, at rates of >20 mm year?1, transfers between closely-spaced faults with acute changes in surface geometry and with diverse rupture characteristics. The trend in seismic hazard assessment since 2016 is to include multifault ruptures universally, but this would be inconsistent with historic events in the Marlborough Tectonic Domain. Consideration of geological structure and history may usefully be incorporated into seismic hazard methodology to evaluate when and where multifault source models are indeed appropriate.

Provenance, paleogeographic and paleotectonic interpretations of Oligocene-Lower Miocene sandstones of the western-central Mediterranean region: A review

The changing nature of detrital signatures in clastic wedges of the Circum-Mediterranean orogenic systems reflect the provenance relations from different source rocks of evolving geo-puzzle terranes, including ophiolite bearing, uplifted continental crust (both shallow to deep crust terranes), volcanic and sedimentary source rocks. We selected here sandstone suites directly occurring over the Mesomediterranean Micropaleoplate during the final stages of closure of the western-southern Tethyan realm. They are unconformably over the internal domains of the Circum-Mediterranean thrust belts, and include Oligocene-to-lower Miocene siliciclastic formations of the Betic Cordillera (As, Bosque, Río Pliego, El Niño, Ciudad Granada, Fuente-Espejos, Alozaina and Viñuela fms), Rif Chain (Fnideq and Sidi Abdeslam fms), and Calabrian terranes (Paludi, Pignolo and Stilo Capo d'Orlando fms). All these sandstone suites range from quartzolitic to quartzofeldspathic detrital modes reflecting close relations with their Paleozoic metasedimentary and plutonic source rocks and their related Mesozoic sedimentary covers. Marked differences have been recognized from western (Betic-Rif) to eastern (Calabria) portions in terms of detritic suites. Detrital suites of the Betic-Rif portions reflect a transition between a craton, transitional and recycled orogenic provenance type. Contrarily, detrital suites of the Calabria portions reflect their transition from transitional continental to basement uplift orogenic provenance reflecting deposition in wedge-top basins during final subduction of the MFB below the MM and the opening of the Mediterranean basin as a backarc.

Tectonic domains in the NW Amazonian Craton from geophysical and geological data

The Amazonian Craton is one of the largest cratonic areas in the world. In Colombia, a major portion of the Craton is covered by Paleozoic to Cenozoic sedimentary rocks and recent deposits. This, in addition to the difficulty of access and dense tropical rainforest, have made the geology of this area to remain relatively unexplored to this date. Most accepted models for the Proterozoic evolution of the Amazonian Craton indicate that it evolved via successive accretion of orogenic belts and crustal terranes around an ancient nucleus, and that tectonic provinces identified in the southern half of the craton, the Guaporé Shield, extend underneath the Amazon River Basin onto its northern exposure, the Guiana Shield. Nevertheless, recent geologic studies in the W Guiana Shield indicate that its evolution may have been different from the W Guaporé Shield, where these accretionary models were formulated. In this work, we used airborne gravity/magnetic geophysical datasets covering the NW portion of the Amazonian Craton, to better elucidate its structure and tectonic evolution. We apply a multiscale edge detection and 3D modeling to identify and delineate major crustal discontinuities and other geological features. Using this approach, we identified six primary geophysical lineaments that are interpreted as possible crustal boundaries. By combining our geophysical interpretation with all the geological, geochronologic and isotopic information available for the region, we propose the presence of the following tectonic domains: Ventuari-Tapajós, Rio Negro-Juruena (which we further subdivide into Atabapo and Vaupés Belts), Apaporis Graben, and Putumayo. Furthermore, a new U-Pb zircon crystallization age of 1227 ± 8/13 Ma obtained from volcanic rocks of the Piraparaná Formation indicates that extensional tectonics along the Apaporis Graben began at least in the late-Mesoproterozoic. This is significantly older than previously thought, and thus entirely transforms the tectonic significance of the Apaporis Graben structures. Our interpretation of structural limits is in excellent agreement with and provides a more accurate location for previously suggested boundaries, which were until now only loosely constrained by the sparse geological and geochronologic information available. This work provides the first regional reconstruction of crustal-scale features of NW South America, improving the understanding of the regional tectonic architecture of NW Amazonian Craton using geophysical methods.

Diachronous Redistribution of Hf and Nd Isotopes at the Crystal Scale—Consequences for the Isotopic Evolution of a Poly-Metamorphic Crustal Terrane

In metamorphic rocks, mineral species react over a range of pressure–temperature conditions that do not necessarily overlap. Mineral equilibration can occur at varied points along the metamorphic pressure–temperature (PT) path, and thus at different times. The sole or dominant use of zircon isotopic compositions to constrain the evolution of metamorphic rocks might then inadvertently skew geological interpretations towards one aspect or one moment of a rock’s history. Here, we present in-situ U–Pb/Sm–Nd isotope analyses of the apatite crystals extracted from two meta-igneous rocks exposed in the Saglek Block (North Atlantic craton, Canada), an Archean metamorphic terrane, with the aim of examining the various signatures and events that they record. The data are combined with published U–Pb/Hf/O isotope compositions of zircon extracted from the same hand-specimens. We found an offset of nearly ca. 1.5 Gyr between U-Pb ages derived from the oldest zircon cores and apatite U–Pb/Sm–Nd isotopic ages, and an offset of ca. 200 Ma between the youngest zircon metamorphic overgrowths and apatite. These differences in metamorphic ages recorded by zircon and apatite mean that the redistribution of Hf isotopes (largely hosted in zircon) and Nd isotopes (largely hosted in apatite within these rocks), were not synchronous at the hand-specimen scale (≤~0.001 m3). We propose that the diachronous redistribution of Hf and Nd isotopes and their parent isotopes was caused by the different PT conditions of growth equilibration between zircon and apatite during metamorphism. These findings document the latest metamorphic evolution of the Saglek Block, highlighting the role played by intra-crustal reworking during the late-Archean regional metamorphic event.

Magmatic evolution of the host magma of plutonic rocks in the Procellarum KREEP Terrane

The origin of the KREEP (K, Rare Earth Element and P)-rich component of the Mg-suite rocks of the Procellarum KREEP Terrane (PKT), one of three major lunar crustal terranes, is unclear. In an attempt to determine its origin, we estimated the composition of host magmas of plutonic rocks, including Mg-suite rocks and evolved rocks, from the PKT (PKT-host magmas) by using secondary ion mass spectrometry analyses of plagioclase. Calculated partition coefficients for Sr, Ba and Ti between plagioclase and melts, taking into account the anorthite content of plagioclase, temperature and bulk rock major-element compositions, were applied to determine parental magma compositions. The PKT-host magmas contained 160–360 ppm Sr, 520–7600 ppm Ba and 1.1–7.0 wt.% TiO2; most of them had higher Ti and Ba concentrations than KREEP basalts and high-K KREEP. We used phase relations based on the Rhyolite-MELTS algorithm to explore the evolution of the PKT-host magmas and KREEP basalts from two bulk silicate moon (BSM) starting compositions, a BSM with chondritic ratios of refractory elements, and a crustal-component-enriched BSM with non-chondritic ratios of refractory elements. We propose a three-stage evolution model. Stage-1: polybaric multi-step fractionation from a BSM magma to form ferroan anorthosite (FAN) crust and an evolved magma as the first KREEP-rich component (M0). Stage-2: assimilation and fractional crystallization (AFC) of M0, early cumulate and FAN associated with deep mantle overturn and impact events to form the PKT-host magmas, Mg-suite rocks and an evolved magma as the second KREEP-rich component (K0). Stage-3: further AFC cycles of K0, Mg-suite rocks or FAN associated with shallow mantle overturn and impact events to form KREEP basalts. This three-stage model for a crustal-component-enriched BSM with non-chondritic ratios of refractory elements (e.g., a sub-chondritic Ti/Ba ratio) reproduced the compositions of both the host magmas of FAN and the PKT-host magmas that fractionated to form the Mg-suite rocks and KREEP basalts of the PKT region. In particular, the model reproduced the high Ti and Ba contents of the PKT-host magmas we estimated from plagioclase composition. Variations of TiO2 and Ba contents (and hence Ti/Ba ratios) of the magmas were critical controls on their evolution.

Assessing the Rotation and Segmentation of the Porcupine Bank, Irish Atlantic Margin, During Oblique Rifting Using Deformable Plate Reconstruction

AbstractPublished plate reconstructions have provided insights regarding the formation of the North Atlantic, in which the motion of the Porcupine Bank, on the Irish Atlantic margin, underlain by orogenic pre‐rift crustal basement terranes, is investigated and restored. Previous reconstructions of the Porcupine Bank mainly relied on potential field data rather than seismic constraints and failed to reveal the role of inherited crustal terranes during rifting and subsequent crustal deformation. In this study, five deformable plate tectonic models with distinct structural inheritance trends are established in GPlates by adjusting a previously published restoration model for the North Atlantic. For each model, driving factors such as the inclusion of the Orphan Knoll, the Flemish Cap poles of rotation, and the motion of the eastern border of the Porcupine Basin are also considered. To assess the validity of deformable plate models, crustal thickness estimates obtained from gravity inversion and seismic data modeling are compared with those calculated via deformable plate models. The preferred deformable plate model proposes the subdivision of the Porcupine Bank into four blocks with each block experiencing poly‐phased rotation and shearing prior to the final continental breakup, implying strong inheritance and segmentation of the Porcupine Bank and Porcupine Basin. The reconstructed paleo‐positions of the Flemish Cap and Porcupine Bank within deformable regions reveal evolving conjugate relationships during rifting, which are assessed using regional seismic transects from both margins. Finally, extensional obliquity between both margins is quantitatively restored, showing time‐variant orientations due to the rotation and shearing of associated continental blocks.

Late Eocene Two-Pyroxene Trachydacites from the Southern Qiangtang Terrane, Central Tibetan Plateau: High-Temperature Melting of Overthickened and Dehydrated Lower Crust

AbstractOrthopyroxene-bearing granitic rock (e.g. charnockite) is relatively rare but provides an excellent opportunity to probe the thermal and tectonic evolution of deep orogenic crust because of its distinct mineral assemblage. Here we present petrological, mineralogical, elemental, and Sr–Nd–Hf–O isotopic data for late Eocene (ca. 36 Ma; zircon U–Pb ages) volcanic rocks exposed in the Ejiu region in the southern Qiangtang Terrane to investigate how the central Tibetan crust evolved to its modern thickness and thermal state. The Ejiu volcanic rocks (EVRs) are trachydacites with anhydrous mineral assemblages (i.e. two pyroxenes, sanidine, plagioclase, and ilmenite, without amphibole and biotite) and geochemical characteristics (e.g. high P2O5 and TiO2) that resemble those of charnockite-type magmatic rocks. Mineral and whole-rock thermometry and hygrometry suggests that the parent magma crystallized under hot (~1000°C) and dry (H2O < 2 wt.%) condition. Besides, the EVRs display adakitic affinities according to their high SiO2 and Al2O3 contents, high Sr/Y, La/Yb, and Gd/Yb ratios, and low Y and Yb contents, without marked negative Eu anomalies. The calculated melts in equilibrium with pyroxenes also display adakitic compositions (e.g. high Sr/Y and La/Yb ratios), indicating that the adakitic compositions of the EVRs did not result from late-stage magmatic evolution. In addition, the melts of the EVRs were saturated in TiO2, as inferred from the high TiO2 contents of these rocks and the presence of ilmenite. An integrated analysis of the geochemical, petrological, and mineralogical data suggests that the EVRs were neither evolutional products nor partial melts of hydrous mafic materials at normal crustal pressures, but were formed by fusion of an eclogitized mafic protolith with residue containing garnet and rutile but lacking amphibole and plagioclase. The whole-rock Sr–Nd and zircon Hf isotope compositions of the EVRs [(87Sr/86Sr)i = 0.7053 to 0.7066; εNd(t) = −1.40 to −0.99; zircon εHf(t) = +1.08 to +5.31] indicate that the parental protolith was relatively juvenile in nature, but also contained some supracrustal materials given the high zircon δ18O values [zircon δ18O = +8.21‰ to +11.00‰]. The above arguments lead us to propose that of partial melting of a previously dehydrated—but chemically undepleted—mafic lower continental crust at high pressure (>1.5 GPa) and high temperature (>1000°C) generated the EVRs. Based on a synthesis of independent geological and geophysical data, we further suggest that the southern Qiangtang Terrane crust of the central Tibetan Plateau was thick, dry, and elevated during the Late Cretaceous to early Eocene time, and that it became abnormally hot owing to the ascending asthenosphere after lithospheric foundering during the middle Eocene.

Progressive accretion recorded in sedimentary rocks of the 3.28–3.23 Ga Fig Tree Group, Barberton Greenstone Belt

Abstract One of the major challenges in early Earth geology is the interpretation of the nature of the crust and tectonic processes due to the limited exposures of Archean rocks. This question is predominantly addressed by numerical modeling, structural geology, geochemical analyses, and petrological approaches. Here we report on the reconstruction of one of the oldest, well-preserved volcano-sedimentary sequences on Earth, the 3.28–3.22 Ga Fig Tree Group in the Barberton Greenstone Belt, South Africa, based on geochronology, provenance, and stratigraphy to provide new constraints on the nature of tectonic processes in the Archean. The Fig Tree basin was asymmetric and the onset of deposition varied across the greenstone belt. The Fig Tree Group is now preserved in east-west oriented bands of fault-bounded structural belts with those preserved in the southern parts of the greenstone belt showing an onset of deposition at 3.28 Ga, those in the center at 3.26 Ga, and those in the north at 3.24 Ga. Stratigraphically, the rocks display a general up-section trend from deeper to shallower-water deposition and/or from finer- to coarser-grained sedimentary rocks. Associated with this up-section stratigraphic trend, the sedimentary rocks show a change in provenance from more regionally similar to more locally variable, and an increase in felsic volcanic activity, especially toward the closure of Fig Tree deposition. The data is consistent with formation of the Fig Tree Group in a compressional tectonic setting by deposition in a foreland basin that experienced progressive accretion of crustal terranes onto a northward prograding fold-and-thrust belt.

Marie Byrd Land lithospheric mantle: a review of the xenolith record

Abstract The Marie Byrd Land (MBL) lithospheric mantle xenolith record comprises over 100 samples from a range of localities spanning both major crustal terranes that comprise MBL: Ross and Amundsen provinces. Coarse granular to porphyroclastic in texture, the xenoliths are predominantly Type I spinel-bearing lherzolites to harburgites, but include rare dunite and pyroxenite examples. Garnet is absent and no hydrous phases, such as amphibole or mica, have been reported to date, although traces of apatite may be present. Characterisation of the lithospheric mantle composition and its evolution however, is hampered by patchy and uneven geochemical analyses across the xenolith suite. Nonetheless, a picture emerges of a heterogeneous lithosphere beneath both Ross and Amundsen Provinces. Previously published and new data reported here are consistent with samples ranging from variably cryptically metasomatized residua from variable (10–25%) degrees of partial melt extraction to refertilized compositions. Limited isotopic data point to a complex history, providing evidence for both ancient Proterozoic lithospheric mantle and preservation of Ordovician events. The Sr–Nd–Pb composition of the sampled lithospheric mantle overlaps the common low-µ isotopic endmember identified in Cenozoic magmatism from MBL and the wider West Antarctic Rift System.

Two-stage crustal growth in the Arabian-Nubian shield: Initial arc accretion followed by plume-induced crustal reworking

Island-arc accretion during the assembly of Gondwana has been widely regarded as the main mechanism for Neoproterozoic crustal growth in the Arabian-Nubian Shield (ANS). However, processes involved to transform the newly accreted juvenile terranes into a typical continental crust remain unclear. Here, we present geochemical, isotopic, and U-Pb geochronological data from the El-Shadli volcanic province (80 km × 35 km and > 10 km thick) in the Eastern Desert of Egypt, which lies in the north-western part of the ANS, and overlies strongly deformed, previously accreted arc terranes. The El-Shadli volcanic province consists mainly of a mafic-felsic bimodal suite and subordinate intermediate rocks that intrude the mafic rocks of the suite. The bimodal suite rocks are tholeiitic, whereas the intermediate rocks have a calc-alkaline affinity. The bimodal suite and the intermediate rocks both yield U-Pb zircon ages of ~ 700 Ma implying they are coeval. High zircon εHf(t) values for the bimodal suite (average εHf(t) = +11.46) as well as the intermediate rocks (average εHf(t) = +9.76) indicate they were either magmatic extractions derived directly from a depleted mantle source, or the products of remelting of juvenile crust. Oxygen isotope data for zircon yield similar δ18O values for both the bimodal suite (average δ18O = 4.94‰) and intermediate rocks (average δ18O = 4.79‰). These are lower than typical mantle values indicating the parental magma in both cases interacted with hydrothermal fluids. Based on the petrological, geochemical, and isotopic data, we suggest that the El-Shadli bimodal suite and the intermediate rocks were produced by reworking of MORB-like and arc-like oceanic lithosphere, respectively, most likely driven by a mantle plume during the break-up of Rodinia. The recognition of the El-Shadli volcanic province as a likely mantle plume-induced post-kinematic magmatism provides a mechanism for the transformation of newly accreted juvenile crustal terranes into a chemically stratified normal continental crust. In addition, such plume events may result in new mantle extractions that are converted into new continental crust.

Magmatic Water Content and Crustal Evolution Control on Porphyry Systems: Insights from the Central Asian Orogenic Belt

Abstract Porphyry systems, the most important reserves of Cu and Mo with significant Au, are genetically linked to the emplacement of hydrous and oxidized intermediate to acidic magmas, in response to temporal and geochemical evolution of crust in orogenic terranes. In this study, comprehensive whole-rock and zircon geochemical and isotopic datasets of intermediate to acid igneous rocks were integrated to characterize the crustal evolution and metallogeny of porphyry deposits in the Central Asian Orogenic Belt (CAOB). The ore-forming porphyries of Cu ± Au ± Mo and Mo deposits have higher but largely overlapped ΔFMQ (the proxy for oxygen fugacity) with those of the barren igneous rocks. However, the ore-forming porphyries of Cu ± Au ± Mo deposits are characterized by distinctly higher whole-rock V/Sc and zircon Eu/Eu* (both are proxies for water content) than barren rocks. Furthermore, the V/Sc ratios positively correlate with the Cu tonnages of calc-alkali porphyry Cu deposits, suggesting that magmatic water contents may yield the first-order control on metal endowment. The general decrease of V/Sc from the Paleozoic to Mesozoic, combined with the negative correlation of V/Sc with K2O and SiO2, also indicates the gradual evolution of crust in CAOB clearly controls the end members of porphyry-type systems (i.e., Cu‒Au and Mo deposits). The crustal residence age (TRes, the time difference between Nd depleted-mantle model age and the crystallization age) of c. 500 Ma from the Nd isotopes is proposed as the threshold distinguishing porphyry Cu ± Au ± Mo (TRes < 500 Ma) and Mo deposits (TRes > 500 Ma) in the CAOB. The coupled zircon Hf isotopes and crustal thickness reveal that the fundamental crustal architecture in the eastern and western CAOB had been built by the Late Permian and Late Carboniferous, respectively, highlighted by the converging trends of εHf(t) commencing at c.250 Ma in the eastern segment and c.300 Ma in the western segment of CAOB, indicating reworking and homogenizing of juvenile crust after collision. In the eastern CAOB, porphyry Cu ± Au ± Mo deposits were formed by juvenile materials in thin island arcs, while porphyry Mo deposits were formed by reworked materials in the thickened orogenic crust after c.250 Ma. In the western CAOB, porphyry Cu deposits in the Balkhash region during the Late Carboniferous were formed in thickened continental crust (generally > 40 km), genetically linked to the culmination of world-wide magmatic addition rates (MARs) triggered by accelerated production of the juvenile crust, in contrast to the porphyry Cu ± Au ± Mo deposits formed in thin island arc (generally < 40 km) during the Early Paleozoic. This study tests the zircon ΔFMQ as proxy for fO2, and zircon Eu/Eu* and whole-rock V/Sc ratios as proxy for water content. It highlights that whole-rock V/Sc ratio is a favorable index for the Cu tonnages of porphyry Cu ± Au ± Mo deposits, and that the distinct porphyry-type mineralization in the CAOB is controlled by the crustal evolution reflected by crustal composition and thickness.

How does the elevation changing response to crustal thickening process in the central Tibetan Plateau since 120 Ma?

When and how the Tibetan Plateau formed and maintained its thick crust and high elevation on Earth is continuing debated. Specifically, the coupling relationship between crustal thickening and corresponding paleoelevation changing has not been well studied. The dominant factors in crustal thickness changing are crustal shortening, magmatic input and surface erosion rates. Crustal thickness change and corresponding paleoelevation variation with time were further linked by an isostatic equation in this study. Since 120 Ma crustal shortening, magmatic input and surface erosion rates data from the central Tibetan Plateau are took as input parameters. By using a one-dimensional isostasy model, the authors captured the first-order relationship between crustal thickening and historical elevation responses over the central Tibetan Plateau, including the Qiangtang and Lhasa terranes. Based on the modeling results, the authors primarily concluded that the Qiangtang terrane crust gradually thickened to ca. 63 km at ca. 40 Ma, mainly due to tectonic shortening and minor magmatic input combined with a slow erosion rate. However, the Lhasa terrane crust thickened by a combination of tectonic shortening, extensive magmatic input and probably Indian plate underthrusting, which thickened the Lhasa crust over 75 km since 25 Ma. Moreover, a long-standing elevation >4000 m was strongly coupled with a thickened crust since about 35 Ma in the central Tibetan Plateau.

Lithospheric density structure of the southern Central Andes constrained by 3D data-integrative gravity modelling

The southern Central Andes (SCA) (between 27° S and 40° S) is bordered to the west by the convergent margin between the continental South American Plate and the oceanic Nazca Plate. The subduction angle along this margin is variable, as is the deformation of the upper plate. Between 33° S and 35° S, the subduction angle of the Nazca plate increases from sub-horizontal (< 5°) in the north to relatively steep (~ 30°) in the south. The SCA contain inherited lithological and structural heterogeneities within the crust that have been reactivated and overprinted since the onset of subduction and associated Cenozoic deformation within the Andean orogen. The distribution of the deformation within the SCA has often been attributed to the variations in the subduction angle and the reactivation of these inherited heterogeneities. However, the possible influence that the thickness and composition of the continental crust have had on both short-term and long-term deformation of the SCA is yet to be thoroughly investigated. For our investigations, we have derived density distributions and thicknesses for various layers that make up the lithosphere and evaluated their relationships with tectonic events that occurred over the history of the Andean orogeny and, in particular, investigated the short- and long-term nature of the present-day deformation processes. We established a 3D model of lithosphere beneath the orogen and its foreland (29° S–39° S) that is consistent with currently available geological and geophysical data, including the gravity data. The modelled crustal configuration and density distribution reveal spatial relationships with different tectonic domains: the crystalline crust in the orogen (the magmatic arc and the main orogenic wedge) is thicker (~ 55 km) and less dense (~ 2900 kg/m3) than in the forearc (~ 35 km, ~ 2975 kg/m3) and foreland (~ 30 km, ~ 3000 kg/m3). Crustal thickening in the orogen probably occurred as a result of stacking of low-density domains, while density and thickness variations beneath the forearc and foreland most likely reflect differences in the tectonic evolution of each area following crustal accretion. No clear spatial relationship exists between the density distribution within the lithosphere and previously proposed boundaries of crustal terranes accreted during the early Paleozoic. Areas with ongoing deformation show a spatial correlation with those areas that have the highest topographic gradients and where there are abrupt changes in the average crustal-density contrast. This suggests that the short-term deformation within the interior of the Andean orogen and its foreland is fundamentally influenced by the crustal composition and the relative thickness of different crustal layers. A thicker, denser, and potentially stronger lithosphere beneath the northern part of the SCA foreland is interpreted to have favoured a strong coupling between the Nazca and South American plates, facilitating the development of a sub-horizontal slab.

New constraints on P–T–t path of high–T eclogites in the Dabie orogen, China

Deciphering the P-T-t path of ultrahigh-pressure (UHP) rocks is crucial for reconstructing the evolution of continental crustal terranes involved in very deep subduction followed by exhumation. Achievieng this goal is especially challenging for UHP terranes which experienced protracted high/ultrahigh temperature (high-T/UHT) exhumation histories, because the widespread anatexis associated to retrograde re-equilibration often obliterate the evidence of UHP metamorphism. In the Dabie-Sulu orogenic belt of central-eastern China, which is the largest UHP terrane in the world, the migmatitic North Dabie complex Zone (NDZ) is the archetype of such UHP terranes experiencing a “slow and hot” exhumation. The NDZ stands out for the widespread anatexis that widely overprinted the traces of eclogite-facies metamorphism, hampering a precise reconstruction of its P-T-(t) evolution.This paper presents the results of a multidisciplinary study integrating P–T pseudosection of phase equilibrium modeling, conventional geothermometry and zircon U–Pb dating with trace element analysis applied on the high–T UHP eclogites from the NDZ. The studied eclogites underwent a multistage metamorphic evolution including post-peak granulite-facies overprinting and subsequent amphibolite-facies retrogression. P–T conditions of the granulite-facies overprinting and the amphibolite-facie retrogression have been estimated at >830 °C, 7–12 kbar and 560–880 °C, 4.3–8.9 kbar, respectively. New zircon U–Pb dating, mineral inclusion and trace element analysis, combined with modeled plagioclase modal variation by pseudosection approach, allowed constraining the ages of prograde metamorphism, eclogite-facies metamorphism, granulite-facies overprinting and amphibolite-facies retrogression of the eclogites at 247–230 Ma, 226–219 Ma, 217–212 Ma and 214–198 Ma, respectively. These results provide new P–T-t estimates and age data for the multiple metamorphic stages during continental collision, contributing to precisely reconstruct the whole P–T–t path of the NDZ.

Updated tectonic terrane boundaries of Botswana determined from gravity and aeromagnetic data

We used existing high-resolution gravity and aeromagnetic data to map the crustal units underneath Botswana. The thick sedimentary cover in this region has always been a challenge to understand the geological and tectonic configuration of unexposed crustal terranes using traditional geological methods. We utilized a standard physical mapping technique to convert previously obtained gravity and aeromagnetic data into apparent density and magnetic susceptibility maps, respectively, of the crustal tectonic terranes. Then, the derived maps were fused into a single apparent physical map to facilitate its geotectonic interpretation. The results showed that most of the tectonic boundaries were inaccurately mapped due to limited availability of high-resolution geophysical information. Moreover, we found that some terranes were missing from the current tectonic map. We used the apparent physical property map and the basement rocks information from literature to significantly update the tectonic map of Botswana, which shows the spatial extent of the geological units beneath the Kalahari sedimentary cover. The new map gives an insight into the geodynamics of Botswana crust.