Mesoproterozoic Rocks Research Articles

Detrital zircon U-Pb-Hf isotopes of late Meso-Neoproterozoic metasedimentary rocks in the South Altyn Tagh: Implications for tectonic affinity of microcontinents in NW China

The Neoproterozoic tectonic correlation between the Central-South Altyn, Qilian, Qaidam, and East Kunlun blocks in northwestern China remains controversial, with competing models favoring separate blocks or a unified single block, and debatable paleo-positions in the Rodinia supercontinent. In this study, we present a systematic provenance study on the late Mesoproterozoic to Neoproterozoic metasedimentary rocks from the South Altyn Tagh. A mica quartz schist sample from the Bashikuergan group yielded a maximum depositional age of 1050 ± 31 Ma. Four paragneiss samples from the Altyn Complex yielded maximum depositional ages of 1106 Ma, 1065 Ma, 1054 Ma, and 870 Ma. Given that the Altyn Complex was intruded by numerous early Neoproterozoic granitoids (ca. 976–900 Ma), we propose that the sedimentary protoliths of the Altyn Complex and the Bashikuergan Group were deposited at two stages, i.e., ca. 1105–975 Ma and after 870 Ma. Provenance tracing indicates that these 1105–975 Ma sediments probably received detritus from the late Mesoproterozoic rocks of Western Australia, East Antarctica, and Central Indian Tectonic Zone in India. In contrast, the detritus of the paragneiss (deposited after 870 Ma) was likely sourced from local regions in the Altyn Tagh orogen. Combined with the comparable magmatic, sedimentary, and tectonic records, we propose that a few microcontinental fragments in northwestern China, including Central-South Altyn Tagh, Qilian, Qaidam, and East Kunlun blocks, constituted a unified block in the early Neoproterozoic and occupied a periphery position of the Rodinia supercontinent with a close paleogeographic affinity to South China and Northwest India.

A gradual Proterozoic transition from an unstable stagnant lid to the modern plate tectonic system

When did plate tectonics begin on Earth, and what preceded it? Published thermo-mechanical mantle evolution models imply that the early history of planets with a composition and size similar to Earth and Venus should be characterized by periodic mantle overturns of 30–100 Myr duration, separated by stable lid phases of 100–300 Myr. This is best described as an unstable stagnant lid because this term captures the Jekyll and Hyde duality of such worlds, which alternate between a stagnant lid sensu stricto phase between mantle overturns, and a mobile lid phase during overturns. Mantle overturn upwelling zones would rework and resurface large tracts of pre-existing Hadean crust and basalt-dominated Archean-style oceanic lithosphere (ASOL). Basal anatexis of ASOL c. 40–50 km thick (or melting within down-drips) could generate tonalite–trondhjemite–granodiorite (TTG) melts and create proto-continental nuclei, while garnet pyroxenite restites delaminate into the mantle. With further reworking, low-K tonalitic rocks would remelt to produce granodiorite and granite, completing the transfer of radioactive elements out of the lower crust. Mantle overturns would generate large-scale lateral currents in the upper mantle that would push against Archean-aged sub-continental lithospheric mantle keels, causing continental drift and orogenesis despite the absence of plate boundary forces such as slab-pull. This is corroborated by the observed displacement of Lakshmi Planum (>1000 km) on Venus, a planet with no arcs nor ridges. Recent models suggest that the Abitibi Greenstone Belt formed as an oceanic tract behind a detached ribbon continent during the partial break-up of the Southern Superior Craton and may be a sample of Archean oceanic lithosphere. The Abitibi Greenstone Belt has c. 50 km of apparent stratigraphy composed of 2–10 Myr mafic–felsic bimodal volcanic cycles that follow assimilation–fractionation trends indicating the contamination of mantle-derived basalts with TTG-like anatectites derived from older basalts. ASOL of this type would be difficult to subduct due to its weakness and buoyancy, but would be fertile and could generate large amounts of second-stage melts. There are no sheeted dykes, precluding a seafloor spreading model, while the absence of basal cumulates or attached mantle means that this type of ASOL should not be called an ophiolite. Archean–Proterozoic unconformities are followed by the deposition of iron formations, clastic and volcanic rocks, which are only rarely affected by sagduction. The increase in siliciclastic input and decreasing sagduction reflect near-global Late Archean emergence of stiffening granitic continents from the oceans due to secular cooling and intra-continental differentiation. Albeit associated with continent-derived siliciclastic debris, many Paleoproterozoic volcanic (and plutonic) rocks resemble Archean rocks geochemically. The similarity of magmatic rocks and hot orogenic styles in the Archean and Paleoproterozoic could imply that the overall geodynamic regime was similar in both. The Siderian–Rhyacian quiet period could therefore represent a stagnant lid phase that followed the 2.5 Ga Archean overturn. When the next mantle overturn ruptured the lid at c. 2.2–2.0 Ga (and again at c. 1.9–1.8 Ga), continents would have been set into motion, forming arcs and ridges. Once initiated, arc and ridge segments needed to multiply and propagate to create a world-girdling system. Mesoproterozoic rocks preserve clear evidence of plate mobility, subduction and orogenesis, but, ophiolites, the geological record of seafloor spreading, are extremely rare prior to 1 Ga. The Earth at 2.0 Ga was probably still largely covered by ASOL, possibly similar to the Abitibi Greenstone Belt, but how and where it was all destroyed and replaced by modern oceanic lithosphere are mysteries. Given the volume of ASOL involved, recognizable by-products of this global-scale reworking process should exist. Voluminous anorthosite–mangerite–charnockite–granite/gabbro suite rocks are mostly of Proterozoic age, requiring either an ephemeral source or a unique process. Trace element inversion models applied to massif anorthosites imply they crystallized from high-La/Yb melts that do not resemble tholeiitic basalts, invalidating the notion that they are flotation cumulates from basaltic underplates. Model anorthosite-forming melts can, however, be explained by high-pressure melting of an ASOL-like basalt source with garnet-bearing residues. I posit that massif anorthosites record the destruction of an ephemeral source (ASOL) at Proterozoic convergent margins. When the last ASOL was crushed between converging continents or consumed by an overprinting arc ( c. 0.8–1 Ga), anorthosite–mangerite–charnockite–granite/gabbro suite rocks ceased to form and the Earth became a modern plate tectonic planet.

Contrasting rare earth element concentrations and mixing behaviors in the St. Lawrence Estuary and Saguenay Fjord

Rare earth elements (REEs) including Yttrium (Y) are commonly used as tracers of estuarine and oceanic mixing. The lanthanide series and yttrium are usually referred to as REYs. The geochemical behavior of REYs in estuarine environments is generally described as being non-conservative, with large-scale removal by particle scavenging. During mixing, partitioning of these elements occurs according to their source function and the stability of natural complexes, with heavy REEs typically forming more stable complexes than light REEs in solution. In this study, we compare the concentrations and partitioning of the 0.7 μm-filtered and 0.05 μm-filtered fractions of the dissolved REYs collected during the summers of 2017 and 2021 in the surface waters (< 3 m) of the St. Lawrence estuarine system (river, estuary and gulf) with those of the Saguenay Fjord, a tributary of the latter that drains the Mesoproterozoic rocks of the Canadian Shield. Whereas REYs do not mix conservatively in the St. Lawrence Estuary (SLE) in the summer, they nearly do so in the Saguenay Fjord (SF). REY concentrations are 2.5 to 6 times greater in the surface waters of the SF than those of the SLE at the same salinity and, in contrast to most estuaries including the SLE, the fjord waters are enriched in LREEs. The 0.05 μm-filtered REY concentrations are positively correlated with dissolved organic carbon (DOC) and chromophoric dissolved organic matter (CDOM) concentrations in the SF but independent of both DOC and CDOM concentrations in the SLE. The CDOM in the fjord differs from that of the estuary as it is more aromatic and has a higher molecular weight. The formation of strong REE-humate complexes stabilizes REY ions in the SF surface waters and impedes their adsorption to and scavenging by solid surfaces during estuarine mixing. The LREE enrichment in the SF surface waters most likely reflects the geology of the fjord's drainage basin, more specifically the exposed Mesoproterozoic granites and gneisses of the Canadian Shield that are enriched in LREE relative to the younger Paleozoic sedimentary rocks exposed along the St. Lawrence Lowlands.

Tectonic evolution of the Aggeneys-Gamsberg Ore District, South Africa, and implications for the geodynamic setting of the Namaqua Sector

Understanding the deformation history of the structurally complex Aggeneys-Gamsberg Ore District (A-GOD), in the Namaqua Sector of the Mesoproterozoic Namaqua-Natal Metamorphic Province of South Africa, is key to discovering additional metasediment-hosted base metal sulphide mineralization. Although several studies have examined the structure of the main ore deposits and outcrops in the district, and others have investigated the timing of sedimentation, magmatism, and metamorphism, no previous studies have focused on integrating these two datasets. We therefore combine structural observations and the resulting relative timeline of events with new and existing zircon U-Pb geochronology, to yield a temporally constrained tectonic history for the district.The regionally recognized Paleoproterozoic D1 deformation is not recorded in the A-GOD, which preserves only Mesoproterozoic rocks. The largely stratiform base metal deposits occur near the top of a sequence of metamorphosed siliciclastic sediments (Wortel and Hotson Formations), deposited after 1261 ± 5 Ma (youngest reliable detrital zircons). These metasediments are tectonically interleaved with the intrusive ∼ 1190 Ma Little Namaqualand Suite (Aroams Gneiss) and younger ∼ 1175–1145 Ma metagranitoids (Achab Gneiss, Hoogoor Gneiss, aplite sills). Magmatism occurred early during D2a deformation, which created kilometre-scale isoclinal folds and thrusts during S/SW-directed contraction, as recorded at the Swartberg and Broken Hill-Deeps deposits. Associated M2a metamorphism is dated at ∼ 1200 Ma (metamorphic zircon rims), but this was not the peak metamorphic episode as previously thought. The sequence was then exhumed, and the unconformably overlying clastics and bimodal volcanics of the Koeris Formation were deposited in local transtensional pull-apart basins at ∼ 1130 Ma. Thrust stacking in the Aggeneys Mountains may have occurred during subsequent D2c contraction, associated with ∼ 1075 Ma M2c metamorphism, but evidence remains cryptic. The entire sequence is affected by kilometre-scale sheath folds that close downwards to the ∼ ENE (as at Gamsberg and the Quarry Fold Nappe), associated with ∼ ENE-trending open, upright folds. These developed during D3a ∼ ENE-directed extensional to constrictional shearing at peak M3 amphibolite-facies conditions, which re-used the gently dipping D2 regional fabric and overprinted it with new ∼ E-W trending D3 linear features. Abundant metamorphic zircon dates suggest an age of ∼ 1025 Ma for D3a and M3. Ongoing extension at retrograde conditions reactivated earlier structures, such as the Swartberg Thrust, as D3b low-angle, ductile–brittle detachment faults. Finally, dextral transform motion created local D4 monoclines and dextral strike-slip faults, possibly associated with ∼ 1000 Ma M4 metamorphic zircon growth and emplacement of a regional pegmatite swarm.The A-GOD has traditionally been fitted into a regional tectonic model involving accretion of distinct terranes onto the Kaapvaal Craton, during the polyphase ∼ 1.2–1.0 Ga Namaquan Orogeny. In contrast, recent studies propose that the entire Namaqua Sector instead occupied a back-arc setting between ∼ 1.35 and < 1.1 Ga. Our results support a hybrid model, involving accretion and translation of previously extended crustal fragments against the craton, with initially the entire Namaqua Sector - but later only the western portion - located in a hot back-arc setting influenced by subducting slab dynamics.

Insights into the evolution of the southeastern Dom Feliciano Belt and its connection to the Pan-African Orogeny based on new U-Pb and Lu-Hf zircon data

The Neoproterozoic supracrustal units of the Dom Feliciano Belt in southern Brazil and eastern Uruguay provide a record of the tectonic evolution of the Brasiliano/Pan-African cycle in terms of sedimentation, deformation, and metamorphism during the western Gondwana supercontinent assembly. From the northern Punta del Este Terrane (Jaguarão Domain) and neighboring Herval Complex were acquired new LA-ICPMS U-Pb and Lu-Hf zircon data from significant samples of metasedimentary rocks and intrusive granites. The central units of the domain are the Arroio Telho and Arroio Grande metasedimentary complexes, which have detrital zircon data ranging in age from 3.2 to 0.7 Ga, with prominent age peaks at 2.2–1.7 Ga, 1.5–1.0 Ga, and 1.0–0.7 Ga. The Herval Complex samples revealed a similar provenance of detrital zircon ages, with two main intervals of 1.5–1.1 Ga and 1.0–0.7 Ga, and also minor Paleoproterozoic (2.0–1.7 Ga) and Archean (2.8–2.4 Ga) ages. The maximum depositional age of the supracrustal complexes is constrained to age populations between 700 and 650 Ma. The mylonitic intrusive peraluminous granites yielded crystallization ages of ca. ∼570 Ma. The Neoproterozoic metamorphic zircon overgrowths range from 680 to 570 Ma. The Lu-Hf data from the meso- to Paleoproterozoic zircons have εHf(t) values ranging from −10 to +10, with the dominance of slightly positive results, and Hf(t) model ages from Paleoproterozoic to Archean. The εHf(t) values in Neoproterozoic zircons are dominantly negative (−15 to +5) with meso- to Paleoproterozoic Hf(t) model ages. These results demonstrate that sedimentary deposition ceased and underwent metamorphism during the development of the Cryogenian/Ediacaran arc-related magmatism of the Dom Feliciano belt. Given the scarcity of Mesoproterozoic rocks in the Mantiqueira Province and nearby cratonic areas, the obtained data suggest that these detrital populations originated in southwestern African rocks. The Congo/Kalahari cratons (Africa) are the most likely source area for Paleoproterozoic and Archean detrital zircons. The geological context and isotopic results of the investigated supracrustal complexes point to a prior extensional tectonic setting during the Tonian (∼800 Ma), with regional deformation and metamorphism linked with the Dom Feliciano Belt accretionary and collisional stages (660–570 Ma).

The stratigraphy and stratigraphic nomenclature of the Goochland Terrane in the piedmont province of east-central Virginia

The Goochland terrane is a structurally isolated crustal block in the eastern Piedmont of Virginia. It is composed of the previously named State Farm Gneiss, Montpelier Anorthosite, Sabot Amphibolite, and Maidens Gneiss, but also includes the Scotchtown Gneiss, Teman Gneiss, and Old Bandana Gneiss which are formally named and defined herein. The eastern part of the Goochland terrane is antiformal and cored by Mesoproterozoic rocks (the State Farm Gneiss and the Montpelier Anorthosite). These basement units are overlain by a late Neoproterozoic to early Paleozoic (Ediacaran to Early Cambrian) saprolitic, metavolcanic, and metasedimentary sequence that sequentially includes the Scotchtown Gneiss, Sabot Amphibolite and Maidens Gneiss. The western part of the terrane is synformal and includes in its core two additional units that overlie the Maidens Gneiss: the Teman Gneiss and the Old Bandana Gneiss. Based on mineralogy and zircon grain morphology, the protoliths of the Maidens, Teman, and Old Bandana gneisses were predominantly sedimentary rocks. The protoliths of the Teman Gneiss and Old Bandana Gneiss were deposited unconformably upon the protolith of the Maidens Gneiss. The eastern and western parts of the Goochland terrane are separated by the Dabneys fault, which has considerable east-side-up vertical offset and possibly also significant transverse displacement. Correlation of the upper part of the Goochland terrane (Teman and Old Bandana gneisses) with the Setters and Cockeysville gneisses in the Baltimore region suggests that the Goochland terrane was left about 135 miles (ca. 220 km) southwest of its original North American location, which was to the east of Baltimore, Maryland. This displacement was caused by the oblique collision of the eastern North American continent with the western edge of the Gondwanan craton during the later Carboniferous (Pennsylvanian) Period.

Paleozoic tectonic setting and evolution of the Central Tianshan Block: Insights from Devonian arc-related granitoids

• Distinct Devonian arc-related granitoids record three phases of magmatism . • Devonian granitoids were derived from remelting of Mesoproterozoic rocks with addition of mantle and juvenile crustal components. • The Central Tianshan Block was a continental arc during the Paleozoic bipolar subduction of the Kangguer oceanic slabs. The Central Tianshan Block of the Central Asian Orogenic Belt (CAOB) contains Precambrian metamorphic basement and voluminous Paleozoic granitoids, but its Paleozoic tectonic setting remains debated. This study presents new geochronological and geochemical data for the granitoids in the Shuangqing district in combination with published data from the Central Tianshan Block to constrain the tectonic setting and evolution of the Central Tianshan Block. Zircon U–Pb dating of the different granitoid plutons identified three phases of magmatism, a foliated granite (408.9 ± 2.6 Ma), a K-feldspar granite (397.7 ± 2.5 Ma), and a granodiorite (porphyry) (ca. 390–388 Ma). These Devonian granitoids show medium- to high-K calc-alkaline affinities, large ion lithophile elements (LILE; e.g., Rb and Ba) enrichment, and high field strength elements (HFSE; e.g., Nb, Ta, and Ti) and Eu depletion, consistent with subduction-related I-type granitoids. They have negative ε Nd (t) values (−5.2 to −3.4), old zircon T DM C ages (1623–1036 Ma), and variable zircon ε Hf (t) values (−3.54 to +5.70), indicating they were mainly sourced from remelting of Mesoproterozoic rocks with variable incorporation of mantle and juvenile crustal components in their magma sources. Combining with published data and this study, we suggest the Central Tianshan Block was a continental arc in the Ordovician to Carboniferous during the bipolar subduction of the Kangguer oceanic slabs. During the Devonian, the subduction of the Kangguer oceanic slabs formed the Central Tianshan continental arc to the south, accompanied by emplacement of multiphase arc-related granitoids in the Shuangqing district.

Structure, metamorphism, and mica40Ar/39Ar thermochronology of the southern Purcell anticlinorium and its transition into the central Kootenay arc, Omineca belt, southeastern British Columbia

The Purcell anticlinorium (PA) and the Kootenay arc (KA) are two adjacent Mesozoic tectonic domains in the southeastern Omineca belt of the Canadian Cordillera. Mesoproterozoic rocks of the Belt-Purcell Supergroup (BPSG) core the PA and have open, upward-facing structures. Deeper levels of the BPSG (Aldridge Formation) preserve a Mesoproterozoic phyllitic cleavage (S1) and biotite zone mineral assemblages. The KA, on the west flank of the PA, has a younger Neoproterozoic through Paleozoic stratigraphy. The central part of the KA, the focus of this study, experienced Jurassic recumbent folding and greenschist facies metamorphism that was overprinted by Early Cretaceous ductile deformation (S2schistosity) and Barrovian metamorphism. The interface between the PA and KA is taken as the easternmost occurrence of the S2schistosity. K–Ar and40Ar/39Ar mica dates in the PA range from 1318 to 240 Ma and show no consistent spatial distribution. They are interpreted to reflect partial resetting of Ar following initial cooling in the Mesoproterozoic, indicating that the PA was never sufficiently buried or heated during younger tectonism to uniformly reset the mica cooling ages. In the KA,40Ar/39Ar dates range from 139 to 55 Ma and reflect previously documented periods of Mesozoic and Cenozoic metamorphism, magmatism, and exhumation. The PA–KA interface occurs above a west-dipping ramp in the Laurentian basement. The thick Mesoproterozoic sequence of turbidites and sills in the PA may have acted as a rigid buttress against which the more ductile rocks of the central KA “piled up” during Early Cretaceous compressional orogenesis.

Tectonic affinity of the Zhusileng–Hangwula Belt in the northern Alxa area: Evidence from the zircon U–Pb ages and Hf isotopic compositions of the Mesoproterozoic (~1.4 Ga) igneous rocks

Precambrian rocks are necessary to understand the tectonic and crustal evolution of the Central Asian Orogenic Belt, especially the tectonic affinities of embedded microcontinents. We conducted a combined analysis of whole‐rock geochemistry, zircon U–Pb geochronology, and Hf isotope on the Mesoproterozoic igneous rocks from the Zhusileng–Hangwula Belt of the northern Alxa area to investigate its tectonic affinity. The zircon ages show that the gneissic granites, metagabbro, and amphibolite formed at 1,433–1,424 Ma, 1,383, and 1,373 Ma, respectively. Furthermore, zircon xenocrysts from the rhyolite in the Zhusileng area exhibit a206Pb/238U ages cluster at about 1.4 Ga. Geochemically, the gneissic granites are high‐potassium calc‐alkaline granitoids and exhibit light rare earth element‐enriched pattern, pronounced significant negative Eu anomalies, negative Nb–Ta anomalies, positiveεHf(t) values (3.2–11.3), and young two‐stage model ages (2.21–1.46 Ga), indicating that they were derived from a magma source of rapid reworking of juvenile mantle‐derived materials with a limited ancient crustal contamination. The FeOT, CaOm and MgO contents of mategabbros and amphibolite are high, but their total rare earth elements are relatively low (TREE = 31.45–32.74). They are characterized by relatively positive zirconεHf(t) values (5.3–11.0), and very young two‐stage model ages (1.98–1.49 Ga), indicating that they were generated from the depleted mantle. Our geochemical data suggest that these Mesoproterozoic rocks in the Zhusileng–Hangwula Belt are consistent with the formation period and tectonic environment of juvenile basement rocks from the Central Tianshan Arc and southern Beishan Orogenic Belt, whereas a Mesoproterozoic correlation with the Tarim Craton is highly unlikely. Thus, we believe that the Zhusileng–Hangwula Belt has an analogous tectonic evolution and a close affinity with the Central Tianshan Arc and southern Beishan Orogenic Belt during the Mesoproterozoic.

Geologic map of southern Panamint Valley, southern Panamint Range, and central Slate Range, California, USA

Abstract This detailed geologic map and supplemental digital data set1 examine and demonstrate the complex deformational history and reactivation relationships of the southern Panamint Valley area (California, USA), from active transtension of the Walker Lane belt, Miocene extension of the Basin and Range, multiple Mesozoic events related to subduction, and Neoproterozoic extension. This collection of map data focuses on the geometry, kinematics, and relative timing of deformation to understand the deformation history and effects of structural reactivation. Andrew and Walker (2009) used these geologic mapping data to palinspastically restore the Fish Canyon area of the Slate Range to overlapping the western Panamint Range at Goler Wash. Neogene extension and subsequent dextral transtension has created a complex network of faults via partial reactivation of Mesozoic and Neoproterozoic structures and has separated the Slate Range from the Panamint Range. The Neogene fault system changes from south to north from dextral strike-slip along the southern Panamint Valley fault to oblique normal slip along the Emigrant fault at a triple junction with the sinistral-oblique normal Manly Pass fault. The Mesozoic deformation history is different in the two ranges across Panamint Valley. The Slate Range was the hanging wall to Jurassic and Cretaceous contractional deformation; this same deformation in the Panamint Range to the east was localized along the western range flank with the majority of the Panamint Range thus being in the footwall to Mesozoic contraction. The western Panamint Range preserves migmatitic fabrics and deformation due to Jurassic contraction and plutonism. The Goldbug fault, along the western Panamint Range, places Paleoproterozoic to Mesoproterozoic rocks over Neoproterozoic to Cretaceous rocks. Jurassic contraction has top-to-the-northeast relative transport and the more discrete Cretaceous thrust faulting in the Panamint Range has top-to-the-east transport. The Butte Valley fault, previously recognized farther north of the map area in the Panamint Range, cuts Late Jurassic rocks and structures. Neoproterozoic to Cambrian sedimentary rocks with top-to-the-northeast contractional deformation occur as relative down-dropped block exposed east of the Butte Valley fault. The Butte Valley fault continues southward and is then deflected by Late Cretaceous thrust faulting on the Goldbug fault. Neoproterozoic deformation is more difficult to discern but is hypothesized to relate to abundant olistostromes mapped within the Kingston Peak Formation in the Panamint Range (i.e., Prave, 1999). This detailed geologic mapping and collection of structural data for the rocks in the southern Panamint Valley area were created using digital in-the-field geographic information systems software running on a field-hardened laptop computer. This map is a simplification of detailed geologic mapping data collected at 1:6000 scales and reduced to 1:20000 scale. Structural data includes kinematic and relative timing of deformation information.

Geologic map of central Panamint Range, California, USA

This detailed geologic map and supplemental digital data set1 examine and demonstrate the complex deformational history and reactivation relationships of the Panamint Range (California, USA), from active transtension of the Walker Lane belt, Miocene extension of the Basin and Range, to multiple Mesozoic events related to subduction, and Neoproterozoic extension. This collection of map data focuses on the geometry, kinematics, and relative timing of deformation to understand the deformation history and effects of structural reactivation. A minor portion of this geologic mapping data was presented in the analysis and figures of Andrew and Walker (2009). The Neogene extension and subsequent dextral transtension deformation has created a complex network of faults via partial reactivation of Mesozoic and Neoproterozoic structures. Structural data show oblique normal slip overprinting earlier normal slip along the western range flank fault of the western Panamint Range. Jurassic and Cretaceous deformation is localized along the western range on the Goldbug fault. The hanging wall of this fault preserves migmatitic fabrics and intense deformation due to Jurassic contraction. The Goldbug fault places Paleoproterozoic to Mesoproterozoic rocks over Neoproterozoic rocks. The Jurassic contraction has top-to-the-northeast relative transport and the more discrete Cretaceous thrust faulting has top-to-the-east transport. A set of Late Cretaceous plutonic rocks and mylonitic gneisses derived from them, occur along the Goldbug fault and demonstrate the reactivated nature of this fault in the Late Cretaceous. New data for the Butte Valley fault show that this fault cuts Late Jurassic plutonic rocks and has normal slip. The Butte Valley fault ends northward at the linked sinistral slip Warm Spring Canyon fault, which was previously interpreted to be an intrusive contact. A previously unrecognized rim syncline structure occurs along the boundary of the Late Jurassic Manly Peak quartz monzonite. Neoproterozoic deformation is difficult to discern due to the overprinting deformations. Numerous Neoproterozoic deformation-related mass wasting deposits can be seen within this formation, including a set of conspicuous allochthonous deposits and clasts of older Beck Spring Dolomite that appear to be frozen in the process of breaking away from intact, normal thickness beds in the Surprise–Happy Canyons divide. This detailed geologic mapping and collection of structural data for the rocks in the central Panamint Range were created using digital in-the-field geographic information systems software running on a field-hardened laptop computer combined with an earlier set of field data that were digitized into the digital georeferenced database. This map is a simplification of detailed geologic mapping data collected at 1:2000–1:6000 scales and reduced to 1:20000 scale. Structural data include kinematic and relative timing of deformation information.

Gold mobilization during prograde metamorphism of clastic sedimentary rocks: An example from the Liaohe Group in the Jiao–Liao–Ji Belt, North China Craton

Abundant gold deposits are distributed in the Jiao-Liao-Ji Belt of the North China Craton. Gold deposits, characterized by quartz lode or disseminated styles of mineralization with extensive alteration of wall rock, of the Jiaodong Peninsula and Liaodong Peninsula are underlain by rocks of similar geological settings characterized by uplifted blocks of metamorphosed Archean and Proterozoic rocks surrounded by relatively unmetamorphosed Mesoproterozoic rocks and Mesozoic granitoid plutons. The huge order-of-magnitude difference in reserves between the Liaodong gold province (500 t) and the Jiaodong gold province (5000 t) attract the attention of geologists. Different C–H-O-S-Pb isotopes and isotopic dating have sparked controversy over the source of the gold deposits. In this study, we conducted integrated analyses of zircon U-Pb dating, bulk-rock compositions, mineralogical compositions and high-precision gold analyses on a series of meta-clastic sedimentary rocks from the Paleoproterozoic Liaohe Group in the Jiao-Liao-Ji Belt, North China Craton, of which the Au analyses reveal Au mobilization during prograde metamorphism. In the Gaizhou Barrovian-type metamorphic zone, the bulk-rock Au contents of samples within various metamorphic zones show a monotonic decrease up-temperature, from 2.09 ppb in the chlorite zone to 0.07 ppb in the sillimanite zone. The evolution in Au content during prograde metamorphism of other formations are identical to that of the Gaixian Formation. The Au concentrations of biotite in different metamorphic zones show little variation ranging from 0.66 to 0.35 ppb, whereas the Au concentrations of muscovite in different metamorphic zones mainly decrease from 2.60 ppb in the chlorite zone to 0.57 ppb in the sillimanite zone with increasing metamorphic grade in samples without metallic sulfides. Coincidently, there is a dechlorination process in biotites during regional metamorphism. Chlorine, released by metamorphism, may have played a vital role in the migration of Au, which could absorb onto muscovite and other minerals, during prograde metamorphism. Given an Au content of 4.67 t/km3 in the chlorite zone rock approximately equivalent to the protolith, metamorphism up to upper amphibolite grade would force the rocks to release 52% of the Au in the Gaizhou Barrovian-type metamorphic zone. Taking into account previous studies in the interpretation of our results, we propose that Precambrian rocks, such as the meta-clastic sedimentary rocks of the Gaixian Formation, may have contributed considerable amounts of Au to the Liaodong gold province. The Liaohe Group releasing large amounts of Au during Paleoproterozoic metamorphism, may thus provide the primary Au enrichment for the Mesozoic gold deposits.

Precambrian metamorphic basement of the southern Lhasa terrane, Tibet

The Precambrian basement of the Lhasa terrane provides important information for understanding the formation and evolution of the Tibetan Plateau. However, due to the paucity of exposure, the properties of the Precambrian basement of the Lhasa terrane remain poorly known. Here we report zircon U–Pb ages, bulk-rock geochemical and zircon Hf isotopic data on the orthogneisses in the Milin area of the southeastern Lhasa terrane, southern Tibet. Inherited magmatic zircon cores of these gneisses yield protolith crystalline ages of 1556–1517 Ma, whereas the zircon rims give metamorphic ages of 1197–1192 Ma, both could be local provenance for the Mesoproterozoic detrital zircons in the Neoproterozoic–Paleozoic strata in the Lhasa terrane. Geochemical data indicate that the protoliths of the Mesoproterozoic rocks are continental arc-like granitoids with bulk-rock εNd(t) values of −3.7 to +0.7 (TDM2 = 1.99–2.34 Ga) and zircon core εHf(t) values of +1.8 to +8.6 (TDM2 = 1.71–2.17 Ga), suggesting their most likely derivation from partial melting of Paleoproterozoic crustal materials. We propose that the Mesoproterozoic gneisses in the Milin area represent the Precambrian metamorphic basement of the Lhasa terrane, likely related to the growth of Columbia supercontinent. Local exposure of the Precambrian metamorphic basement may be caused by crustal rapid uplift and erosion in the late Cenozoic and their inhomogeneous distribution.

Provenance transition from the North China Craton to the Grenvillian orogeny-related source: Evidence from late Mesoproterozoic-early Neoproterozoic strata in the Liao-Ji area

Late Paleoproterozoic to early Neoproterozoic strata in Liao-Ji area (eastern Liaoning-southern Jilin) on the eastern margin of the North China Craton (NCC) provide a significant record of sediment provenance transition from the NCC to the Grenvillian orogeny. A >3000-m-thick sequence of fluvial, margin marine, and offshore marine sediments were preserved, including the Madaling, Baifangzi, Yongning, Diaoyutai, Nanfen and Qiaotou formations. To evaluate the relationship of Rodinia sediment delivery to this area, we synthesize 3177 geochronology data from the Liao-Ji area of which 1499 detrital zircon and 319 monazite ages from 17 sample analyses are newly reported herein. The results indicate that the Madaling, Baifangzi, Yongning, Diaoyutai (not including Dalian samples) and Nanfen Formations have two major peaks at 1.8 Ga and 2.5 Ga. The depositional ages of the formations are 1.27–0.92 Ga, as constrained by the ages of the youngest detrital zircons and later mafic intrusions. In the Madaling/Baifangzi/Yongning period, Longgang and Liaonan blocks and the late Paleoproterozoic Jiao-Liao-Ji belt were dominant in deposition. During the Diaoyutai-Nanfen period, grains derived from Mesoproterozoic rocks began to accumulate in topographically low areas, such as the Fuzhou Gulf. In the Qiaotou period, Mesoproterozoic grain-derived sources dominated the deposition. Combining the studies of late Mesoproteorozoic-early Neoperoterozoic successions from the Xuhuai and Jiaobei areas, the detrital zircon spectra in the Liao-Ji area document the transition from the initial NCC to a distant Grenvillian orogeny related provenance (Laurentia, Siberia, SF-Congo and Baltica have been excluded) from possible south/southeast.

Geochronology of monazite related to REE, Nb–Ta and U–Th bearing minerals from Mesoproterozoic anorogenic magmatism in the E-Colombian Amazonian Craton: Links to mantle plume activity in the Columbia (Nuna) supercontinent

U–Th–Pb geochronology of REE-bearing minerals such as monazite and other phosphates has become a precise tool to date mineralization events through geological time. In the E-Colombian Guainía department, from 3 colluvial sites over western Guiana Shield Mesoproterozoic rocks, Nb–Ta, U–Th and REE bearing minerals probably of pegmatitic origin were sampled including up-to-0.5 cm long monazite crystals. The monazite pan concentrate samples from sites near to the indigenous communities of Barranquilla, San Jose and Chorro Bocón (Espina hills) along the Cuyari, Guainía, and Inírida rivers respectively, were analyzed geochemically and geochronologically using LA-ICP-MS and EPMA. The monazites obtained from the 3 sites are rather similar with an approximate composition Ce0.44La0.16Nd0.16Sm004Y0.03Gd0.02Dy0.01Pr0.05Th0.08Pb0.01Ca0.05Si0.05P0.91O3.94. Their EPMA and LA-ICP-MS ages suggest at least two mineralization events, a first one between 1528 and 1523 Ma in the Barranquilla and San Jose sites, related to the emplacement of rapakivi granites outcropping along the Cuyarí River and the Tabaquen rapakivi granite near the Guainía River, both emplaced ~1545 Ma ago. The second mineralization event from 1390 to 1343 Ma recorded in the Espina-hills is in the age range of the Matraca and Danta rapakivi granites that crystalized between 1400 and 1340 Ma ago. These different anorogenic rapakivi granites were emplaced between 1550 and 1300 Ma into Mesoproterozoic basement rocks and were probably caused by intra-plate processes associated with mantle plume activity within the Columbia (Nuna) supercontinent.

Geochemistry, geochronology, and zircon Hf isotopic compositions of felsite porphyry in Xiangshan uranium orefield and its geological implication

Recently, a new kind of volcanic rock, felsite porphyry, has been revealed by drilling in Xiangshan area, Jiangxi Province, China. To better understand petrogenesis and magmatic evolution sequence of the Xiangshan volcanic-intrusive complex, we studied systematic petrology, geochemistry, LA-ICP-MS zircon U–Pb dating, and Hf isotope results of the felsite porphyry. Results show that the felsite porphyry has similar geochemical characteristics to the porphyroclastic rhyolite, which is the predominant lithology of Xiangshan uranium orefield. Felsite porphyry and porphyroclastic rhyolite have high SiO2, Al2O3, and K2O contents, low Na2O, and MgO contents, and slightly negative Eu anomalies. Moreover, these rocks are relatively depleted in large ion lithophile elements (K, Ba, and Sr) and are enriched in high field strength elements (Th, Zr, and Hf). LA-ICP-MS zircon U–Pb dating of the felsite porphyry yielded a crystallization age of 132.2 ± 0.9 Ma, which is coeval to that of the porphyroclastic rhyolite. These ages signified that Xiangshan volcanic-intrusive complex formed in the Early Cretaceous, during which the entire South China was in the back-arc extension tectonic setting related to the subduction of the Pacific Plate under the Euroasian Plate. In-situ zircon Hf isotope data on a felsite porphyry sample show eHf(t) values from − 8.82 to − 5.11, while the Hf isotope two-stage model age (TDM2-Hf) ranges from 1513 to 1747 Ma. Combined with petrological, mineralogical, geochemistry, and geochronology results of the felsite porphyry, it is concluded that the felsite porphyry in Xiangshan might be originated from the partial melting of the Mesoproterozoic ancient metamorphic rocks, with possible input of small amounts of mantle materials.

Petrogenesis and tectonic setting of the early-middle triassic subduction-related granite in the eastern segment of East Kunlun: evidences from petrology, geochemistry, and zircon U-Pb-Hf isotopes

ABSTRACT The subduction process of the Paleo-Tethyan Ocean is still under controversy. We report zircon U-Pb ages and geochemistry (major and trace elements, and Hf isotopic compositions) for the Xiangjiananshan granitic plutons in the eastern segment of the East Kunlun Orogenic Belt, northern Tibetan Plateau, to constrain the subduction-related granitoids in its petrogenesis and to reconstruct the subduction prosess of the Buqingshan-A’nyemaqing Ocean. Zircon U-Pb dating yields coeval ages of 246.6 ± 4.1 Ma for granodiorite and 246.4 ± 3.9 Ma for biotite monzogranite of the Xiangjiananshan granitoid pluton. The granitoids include granodiorites, biotite monzogranites, and pseudo-porphyritic monzogranites. The granitoids have high contents of SiO2 (66.60–73.19 wt.%) and Al2O3 (13.28–16.38 wt.%), consistent with the high-K calc-alkaline series. The A/CNK ratios range from 0.98 to 1.02, indicating that the granodiorite is metaluminous and weakly peraluminous. The mafic microgranular enclaves (MMEs) in granodiorite and biotite monzogranite have a restricted SiO2 range (57.24–60.03 wt.%), and have higher Fe2O3 T, and MgO contents than the host granitoids, as well as higher total rare earth element (REE) concentrations. All samples are depleted in high field strength elements (HFSEs; e.g., Nb, Ta, Ti and P) and heavy rare earth elements (HREEs), and are enriched in large ion lithophile elements (LILEs; e.g., Rb, Nd and La) and light rare earth elements (LREEs). Zircons from the pseudoporphyritic monzogranite yield two-stage model ages of 1137.32 to 1588.12 Ma, together with εHf(t) values from −5.01 to +2.11. These results indicate that the parental magma of pseudoporphyritic monzogranite was generated by partial melting of Paleoproterozoic to Mesoproterozoic rocks of the lower crust with contamination of ancient crustal material. Field and petrological observations, together with elemental and Hf isotopic data, indicate that the Xiangjiananshan granitic plutons was formed by magma mixing of mafic and felsic magmas. The felsic magma was derived from partial melting of the lower crust, while the mafic magma was derived from partial melting of an enriched lithospheric mantle metasomatized by slab-derived fluid due to the subduction of the Buqingshan-A’nyemaqing Ocean. Therefore, we propose that the northward subduction of the Buqingshan-A’nyemaqing ocean north to the Bayan Hara block under East Kunlun initiated in the Middle Permian (ca. 270 Ma), yielding calcium metaluminous arc magmatic rocks with I-type granite characteristics at East Kunlun, including the XG pluton in this study. Subsequently, the Buqingshan-A’nyemaqing ocean closed and resulted in collision of the Bayan Hara and East Kunlun blocks from 240 to 230 Ma.

Mineralogy and U/Pb ages of rutile-(Nb,Ta) related to cassiterite, and columbite from Mesoproterozoic rocks of the Amazonian Craton near Cachicamo, Colombia

Para resolver la pregunta si megacristales de rutilo-(Nb,Ta) en la Comunidad Indígena de Cachicamo (NE-Vichada) provienen de rocas del Granito Rapakivi de Parguaza se extrajeron rutilos de una coraza ferruginosa superficial o duricrust compuesta por (hematita, limonita, ± magnetita, ilmenita, circón, fragmentos de roca), desarrollada sobre un perfil laterítico del basamento granítico, que fueron analizados junto a suelos y concentrados. Mediciones simultáneas de densidad y FRXp de &gt;250 rutilos revelaron variaciones fuertes desde rutilos livianos (~3,4 g/cm3), de casi puro TiO2, hasta rutilos-(Nb,Ta) de 5,9 g/cm3 enriquecidos en Nb, Ta, Fe, Mn, Sn, V y W. Metalografía, MEB-EDX, FRX y DRX revelaron inclusiones de hasta 0,2 mm compuestas de ~17% columbita-(Fe) y 4% casiterita en una matriz de ~78% rutilo-(Nb,Ta). Valores EPMA (72% TiO2, 11% Nb2O5, 9% Ta2O5, 6% Fe2O3, 1,5% SnO2) confirman junto con bajos contenidos de W y Mn una afinidad pegmatítica - pneumatolítica. Mediciones ICP-MS indicaron suficiente U para probar dataciones en la Universidad Rennes 1, donde, usando estándares R10 y R19, diez rutilos-(Nb,Ta), arrojaron edades de cristalización U/Pb concordantes de 1512±12 Ma. Estas edades coinciden con observaciones de campo, según las cuales los rutilos-(Nb,Ta) de Cachicamo cristalizaron en el Granito de Parguaza hace 1500-1550 Ma en Venezuela, donde depósitos secundarios contienen mineralizaciones comparables (sub-?) económicas de casiterita, rutilo-(Nb,Ta), columbita-tantalita y monacita. El análisis de minerales pesados (circón, ilmenita, magnetita) muestra que los rutilos-(Nb,Ta) se limitan al duricrust del horizonte A, donde circones detríticos de 1525 Ma son coetáneos al Batolito de Parguaza en Venezuela (1500-1550 Ma), mientras los circones del horizonte C (regolito) con 1415 Ma tienen edades similares al Granito Rapakivi Parguaza local (1340-1402 Ma) en estas partes distales del Batolito en Colombia. Así, los rutilos-(Nb,Ta) de Cachicamo no se pueden haber formado in-situ sino que experimentaron un transporte post-exhumación por decenas de kilómetros atravesando lo que actualmente es el valle del río Orinoco.

Paleomagnetism and Age Correlation of the Mesoproterozoic Rocks of the Udzha and Olenek Uplifts, Northeastern Siberian Platform

Abstract—This paper presents the results of paleomagnetic studies of the Proterozoic sedimentary and igneous rocks composing the structure of sections of the Olenek and Udzha uplifts. Carbonate rocks of the upper unit of Khaipakh formation have been studied within the Olenek uplift, and terrigenous and volcanic-sedimentary rocks of the Udzha and Unguokhtakh formations and the Early Mesoproterozoic igneous complex have been investigated within the Udzha uplift. Within the Olenek uplift, the carbonate rocks of the upper unit of the Khaipakh formation have been studied; within the Udzha uplift–terrigenous and volcanogenic-sedimentary rocks of the Udzha and Unguokhtakh formations, and the Early Mesoproterozoic magmatic complex. The results of the paleomagnetic study (1) indicate that the upper unit of Khaipakh formation of the Olenek uplift and the Udzha formation of the Udzha uplift have different sedimentation age, which contradicts the currently accepted correlation scheme of the Proterozoic sections in the northern Siberian platform; (2) support the existence of two stages of the Proterozoic magmatism in the territory of the Udzha uplift, with a more ancient stage having an age of ~1500 Ma according to the paleomagnetic data; (3) show that from ~1500 to 1110 Ma ago, the Siberian platform was located in the equatorial and subequatorial latitudes.

Organic geochemical compositions of Mesoproterozoic source rocks in the Yanliao Rift, Northern China

Organic-rich Mesoproterozoic rocks from the Xiamaling and Hongshuizhuang formations, with a moderate thermal maturity within the oil window, are well preserved in the Yanliao Rift, Northern China. Their organic geochemical compositions, especially biomarkers, organic matter origin, and depositional environment, are discussed in detail using GC, GC–MS and GC-MS-MS techniques. The results indicate that the Xiamaling and Hongshuizhuang samples share similar molecular compositions, which are distinct from those of Phanerozoic bitumens, suggesting a unique microflora in the North China sea during the Middle Mesoproterozoic (1.45–1.30 Gyr). The biomarkers in the saturated fraction were dominated by 13α(n-alkyl)-tricyclic terpanes (13αNATTs) and drimanes. The 13αNATTs appeared to be diagnostic biomarkers for special, ancient microbes in the Proterozoic, as they were only detected in Meso–Neoproterozoic bitumens and were absent in Phanerozoic sequences. Abundant drimane series, including extended drimanes, may be sourced from the ring C cleavage of 13αNATTs or may directly originate from the same precursor of 13αNATTs, based on the observation that the concentrations of drimanes and 13αNATTs exhibited a good correlation. Elevated C19 and C20 regular tricyclic terpanes together with abundant rearranged hopanes, which are usually typical fingerprints for terrestrial rocks, are distinct features of the Xiamaling and Hongshuizhuang marine rock samples. These compounds are considered to be derived from unique prokaryotes that were present in the Mesoproterozoic sea, which could synthesize abundant precursors of rearranged hopanes and low molecular tricyclic terpanes. Pristane (Pr) and phytane (Ph), mainly originated from the phytol side-chain of chlorophyll, were only present in very low abundances, even in the low maturity and organic-rich Xiamaling oil shales (total organic carbon >20%). This may indicate a contribution of phototrophs with other molecular proxies, rather than phytanic lipids in their chlorophylls. An absence or trace amounts of steranes in the samples may reveal a low contribution of eukaryotes, although previous studies have found a wide occurrence of eukaryotes in the Xiamaling and Hongshuizhuang formations, as well as in even more ancient rocks in the Yanliao Rift. Low Pr/Ph ratio values (<1.0), together with an absence of dibenzothiophenes and aryl isoprenoids, may indicate a common anoxic environment with relatively low sulfate concentrations and a low contribution of green sulfur bacteria. Several maturity indictors, such as Tmax, C31 22 S/(22 S + 22 R), and the methylphenanthrene index, all suggest an early oil-window thermal maturity for the Xiamaling rocks and a middle oil-window maturity for the Hongshuizhuang rocks.