Shoufa Lin

The Corner Brook Lake block in the Newfoundland Appalachians: A suspect terrane along the Laurentian margin and evidence for large-scale orogen-parallel motion

Available geochronological (U-Pb and Ar/Ar) and geological data indicate that the Corner Brook Lake block of the Humber zone in the western Newfoundland Appalachians has unique characteristics. Grenvillian ages (ca. 1.0 Ga), which are typical for the Laurentian Appalachian margin, are absent in the crystalline basement to the Corner Brook Lake block. This makes it unlikely that the block is underlain by true parautochthonous Humber margin basement. However, the lithological makeup of its late Neoproterozoic–Early Cambrian sedimentary cover and detrital zircon populations indicate a Laurentian-type provenance. In addition, there is no geochronological or other geological evidence for a Middle Ordovician tectono-thermal event in the Corner Brook Lake block, suggesting that it escaped the penetrative tectono-thermal effects of the Taconic collision present elsewhere in the Laurentian realm. Instead, the block underwent strong regional deformation and associated peak metamorphism during the Silurian Salinic orogeny. Combined evidence, including the absence of Grenvillian ages, the presence of late Neoproterozoic ca. 600 Ma granitoid plutons, the regionally distinct early Paleozoic tectonic history of the Corner Brook Lake block, and its fault-bounded nature, implies that the block represents an allochthonous terrane. Available data indicate that significant orogen-parallel movement of the block (possibly 400 km or more) could have taken place during the Appalachian orogeny. The possibility of large-scale strike-slip tectonics, in addition to the well-documented convergent motions, has significant implications for the tectonic interpretation of the early Paleozoic evolution of the Newfoundland Appalachians.

The Middle Ordovician to Early Silurian voyage of the Dashwoods microcontinent, West Newfoundland; based on new U/Pb and 40Ar/39Ar geochronological, and kinematic constraints

The Dashwoods microcontinent is an important tectonic segment in the peri-Laurentian setting of the Newfoundland Appalachians. In order to better understand the tectonic history of Dashwoods during the Ordovician Taconic orogeny, we have undertaken field mapping, microscopic studies, and U-Pb and 40Ar/39Ar geochronological studies along the northern (Little Grand Lake Fault; LGLF) and western (Baie Verte Brompton Line - Cabot Fault Zone; BCZ) boundaries. Oblique-dextral ductile deformation in the BCZ occurred from late Middle Ordovician into the Early Silurian, based on the presence of a late syn-tectonic pegmatite dike (455 ± 12 Ma) and a foliated granodiorite sheet (445.8 ± 0.6 Ma). Deformation is coeval with oblique-sinistral accretion along the eastern margin of Dashwoods, which means that Dashwoods and its Notre Dame Arc had a southward translation with respect to the Laurentian margin and the then-present Iapetus Ocean during the Late Ordovician. Dextral movement along the BCZ continued after the collision of Dashwoods with the Laurentian margin. Deformation along the Little Grand Lake Fault is bracketed between 463 ± 5 Ma and 440 ± Ma. These ages combined with other geological arguments indicate that motion probably took place during the Late Ordovician to earliest Silurian contemporaneous with the southward translation of Dashwoods. A possible explanation is that the Snooks Arm arc moved independently from and faster southwards than the Notre Dame Arc with its Dashwoods infrastructure, thereby underthrusting the Dashwoods along the Little Grand Lake Fault. Our new U-Pb geochronological data, including a muscovite granite (463 ± 5 Ma), a schistose muscovite granite (459 +17/-21 Ma), and a tectonized tonalite (458 ± 20 Ma), add to the geochronological database of the voluminous second phase of the Notre Dame Arc. Additionally, in all-but-one of our U-Pb samples, inherited grains of Mesoproterozoic (circa 1.0 Ga) age have been obtained. Their regional presence fortifies the possible relationship of the Dashwoods microcontinent with the Long Range Inlier in western Newfoundland. Furthermore, it introduces a potential link with the Blair River Inlier in Cape Breton Island.

First Direct Evidence of Pan-African Orogeny Associated with Gondwana Assembly in the Cathaysia Block of Southern China

Metamorphic zircon from a hornblendite in the South China Block (SCB) yield U-Pb age of 533 ± 7 Ma and Hf model ages from 900 to 1200 Ma. Geochemical and isotopic characteristics indicate that primary magma of the hornblendites was probably derived from an enriched asthenospheric mantle source. This Late Neoproterozoic–Cambrian (Pan-African) metamorphic event provides the first direct evidence that the SCB was an integral part of the Gondwana assembly. Combined with available geological data which show that the SCB has great affinity with India or Australia, the Pan-African metamorphic event most likely belongs to the eastern Kuunga orogeny. We propose that the SCB was located at the nexus between India, Antarctica and Australia along the northern margin of East Gondwana, with the Cathaysia Block connecting western Australia whereas the Yangtze Block facing northern India at ca. 533 Ma.

Structure and geochronology of the Tongbai complex and their implications for the evolution of the Tongbai orogenic belt, central China

ABSTRACT The Tongbai orogenic belt has an overall antiformal geometry and the hinge of the antiform is sub-horizontal and trends NW–SE. The Tongbai complex (TBC) in the core of the antiform is bounded by the S-dipping Yindian–Malong shear zone in the south, the sub-horizontal Taibaiding shear zone at the top and the N-dipping Hongyihe–Tongbai shear zone in the north. The three shear zones have dextral, top-to-NW and sinistral movement, respectively. They are parts of a single shear zone, termed the Tongbai shear zone, that has a uniform top-to-NW sense of shear. Three samples of deformed granitoid (mylonite or protomylonite) from the shear zone have U–Pb zircon ages of 145 ± 6 Ma, 142 ± 2 Ma and 131 ± 6 Ma, respectively. An L-tectonite in the TBC yielded a metamorphic age of 137 ± 8 Ma and a migmatite an age of 137 ± 1 Ma. The Tongbai shear zone is intruded by undeformed Early Cretaceous granite and dykes and deformation in the shear zone is constrained to ca. 140–135 Ma, synchronous with metamorphism and migmatization in the TBC. Early Cretaceous magma emplacement and the associated uplift modified the TBC into a gentle antiform and the uplift may have continued to ca. 102–85 Ma. Similar geometry and kinematics have been documented in the Dabie orogenic belt to the east, which suggests that the Central Orogenic Belt in China probably experienced a uniform orogen-parallel extension and top-to-NW shearing in the ductile lithosphere in the Early Cretaceous.

Elucidating tectonic events and processes from variably tectonized conglomerate clast detrital geochronology: Examples from the Permian Hongliuhe Formation in the southern Central Asian orogenic Belt, NW China

This article expands upon detrital zircon geochronology with a sampling and analysis strategy dating variably tectonized granitoid conglomerate clasts. Its purpose is to elucidate details of the provenances tectonomagmatic history from deformation-relative age distributions. The method involves bulk samples of clasts, sorted based on the degree of internal ductile deformation. Isolating granitoid clasts, we divide them into three subsets: undeformed, slightly deformed, and deformed. Laser ablation, inductively coupled plasma mass spectrometry U-Pb geochronology is performed on zircon separates of each subset. Our case study, involving the Permian Hongliuhe Formation in the southern Central Asian Orogenic Belt, analyzes each of the three clast subsets, as well as sandstone detrital samples, at three stratigraphic levels and yields a profile of the unroofed provenance. Clast age distributions exhibit different, wider-age spectra than sandstone samples, an effect of proximity to the respective provenance. They indicate a minimal lag time, implying rapid exhumation rates, whereas sandstone data alone would indicate a 90 Myr lag time. Early Paleozoic arc-building episodes appear as Ordovician peaks in sandstone data and Silurian-Devonian peaks in clast data, indicating a younging of magmatism toward proximal provenance. A magmatic hiatus starts in the Devonian, correlating with the latest age of deformed clasts, interpreted as timing of collisional tectonics. The detailed age spectra provide regional tectonic context and interpretation of processes, as well as more robust provenance interpretation than could be determined from sandstone samples alone. The variably tectonized clast detrital geochronology method removes human sampling bias and the practical limits of studying regional granitoid distributions.

Petrogenesis and mantle source characteristics of Cenozoic alkaline diabase, Jiangxi Province, southeastern China

Major element, trace element, and Sr–Nd–Pb isotopic compositions of Cenozoic diabase in southeastern China provide insights into the nature of their mantle sources and processes. The diabases are alkaline in lithochemistry (Na2O + K2O = 4.37–5.19 wt.%) and have overall oceanic island basalt-like trace element patterns, without negative Nb–Ta anomalies. In addition, they are characterized by lower La/Nb (<1.5) and La/Ta (<22), and higher Ce/Pb (>15) and Nb/U (>30) ratios, indicating an origin in the asthenospheric mantle. The relatively lower 143Nd/144Nd (0.512632–0.512648) and 206Pb/204Pb (18.20–18.22), but intermediate 87Sr/86Sr (0.7061–0.7063) ratios of the diabases are similar to enriched mantle type 1, suggesting crustal contamination or mixing with metasomatized lithsopheric mantle. However, the low Th and U contents and lack of correlations of Nd isotope compositions and MgO preclude significant crustal contamination. Alternatively, the moderate TiO2 contents (2.01–2.09 wt.%) and high Cr concentrations (>240 ppm) suggest interaction between asthenosphere-derived melts and metasomatized lithospheric mantle. Petrological modelling suggests that the diabases were generated from a low degree (~3–5%) of partial melting of lherzolite with ~2–3% garnet. Jiangxi diabase was generated in a within-plate extensional regime, probably related to the far effect of the Himalaya–Tibetan orogen.

A Uniform Orogen-parallel Extension System of the Shear Zones in the Tongbai-Dabie Orogenic Belt, Central China

Large-scale magmatism affected the Tongbai-Dabie orogenic belt during post-collisional lateral tectonic extension in the Cretaceous, which was suggested to account for the widespread deformation and migmatization in the Tongbai-Dabie complexes. However, it cannot explain the most deformations in the shear zones. The northwest-southeast shear zones are developed around or wrapped the Tongbai-Dabie complexes. They play an important role for the interpretation of the tectonic evolution of the Tongbai-Dabie orogenic belt. By a systematically observation and description of the geometry and kinematics of these shear zones, we found that the shear zones to the north dip NE and have a uniform sinistral shear sense, the shear zone to the south dips SW and has a uniform dextral shear sense, and the shear zones at the core are sub-horizontal and have a uniform top-to-NW sense of shear. Combining with the comparison of previous and our geochronological studies, we interpret these associations as indicating that these shear zones were originally a single, more flat-lying and subhorizontal shear zone with a uniform top-to-NW shear sense before the folding-doming of the TongbaiDabie complexes and suggest that the Tongbai-Dabie orogenic belt experienced a uniform top-to-NW orogen-parallel extension in the ductile lithosphere before the widespread magmatism in the Cretaceous.

Deformation of the Shirenshan Block in the North Qinling

The Qinling orogenic belt is a typical complex continental orogenic belt which has experienced multiperiod tectonic evolution and where some important tectonic belts formed. The Luoluan fault is one of the most important belts, which is the boundary fault of the North China Plate and the Qinling orogenic belt. The Shirenshan block is located in the north section between Luanchuan and Fangcheng of the Luoluan fault. The north part is undeformed Yanshanian monzogranites; the middle part is Proterozoic granite gneiss, and in the south it is mainly gneiss and migmatite of the Taihua Group from late Archean to early Proterozoic. Previous research suggested that the Shirenshan block is an ancient geological body, which is remelted by the subduction shear of Luoluan fault. The granite has been squeezed into the shear zone and it occurred at about 380–400Ma. However, sufficient data show that the Shirenshan block has zircon ages of 110–130Ma, which is inconsistent with the previous studies. Therefore, it is necessary to uncover the formation of Shirenshan block and its relationship with Luoluan fault, and it could provide basis for the study of Mesozoic intracontinental orogeny of the north Qinling.

Synchronous vertical and horizontal tectonism during the late stage of Archean cratonization: An important process in gold mineralization?

Both vertical and horizontal tectonism played an important role in Archean tectonic evolution. These two processes have independent driving forces and should not be mutually exclusive. In the Superior Province in Canada, there is convincing evidence for synchronous vertical and horizontal tectonism at the late stages of Archean cratonization, and horizontal shearing (a result of horizontal tectonism) is concentrated in synclinal keels (a result of vertical tectonism). The Timiskaming-type sedimentary rocks were deposited in the keels during this process. The synclinal keel-shear zone association provided a link between the upper crust and the lower crust or mantle, and might have served as a conduit for mineralizing fluids and magma that were generated in the crust and/or mantle during the process. Such a process in the late stage of Archean cratonization can readily explain the common association of gold deposits with greenstone belts in synclinal keels, shearzones, late felsic to intermediate intrusions and Timiskaming-type sedimentary rocks.