Arc-continent Collision Research Articles

Origin and tectonic implications of the early Middle Triassic tuffs in the western Yangtze Craton: Insight into whole-rock geochemical and zircon U-Pb and Hf isotopic signatures

The tuff layer steadily distributed in the western Yangtze Craton is a regional marker for the Early–Middle Triassic (Olenekian–Anisian) boundary. Integrated studies of whole-rock geochemistry and zircon geochronology reveal that the parental magma of the tuff is of felsic composition, generated via volcanic eruptions in the continental arc setting along the southwestern margin of the Yangtze Craton at ~247 Ma. The syn -depositional zircons in the tuff layers and in the coeval igneous rocks exhibit uniform negative ε Hf ( t ) values of approximately −20 to −6, indicating the significant input of crustal materials. Massive crustal assimilation implies a large-scale remelting of the pre-existing continental crust, triggered by the intensive arc–continental collision during that time. This intensive collisional tectonomagmatic event is an important signal for the closing of the eastern Paleo-Tethys branch and points to the initial collision of the Indochina Block and the Yangtze Craton. Xenocrystic zircons in the Precambrian-age offer insight into the early evolution of the southwestern Yangtze Craton, which shows episodic crustal growth and reworking of ~2.9–2.2 Ga, crustal reworking of ~1.9–1.7 Ga, rifting-related tectonomagmatism of ~1.6–1.4 Ga, and crustal growth of ~1.1–0.8 Ga. The older crystallization ages (3.4–2.5 Ga), lower initial 176 Hf/ 177 Hf values (0.2805–0.2813), negative ε Hf ( t ) values (−15.7 to −1.6), and two-stage Hf model ages (3.9–2.8 Ga) of zircons further suggest that an unexposed Archean basement exists beneath the southwestern Yangtze Craton. Significant contributions of mantle-derived materials occur at ~1.3–1.0 Ga, implying that the southwestern Yangtze Craton may not be involved in the Grenvillian-aged continental–continental collision orogeny. Comparable age spectrums and Hf isotopic data suggest that the Paleoproterozoic igneous rocks are a non-negligible provenance for Proterozoic metasedimentary in the Kangdian region. • Tuffs erupted at ~247 Ma are the products of volcanism in the continental arc setting. • The source region probably located along the southwestern margin of Yangtze Craton. • ~247 Ma is a key temporal node for the closing of a branch of eastern Paleo-Tethys.

Inherited lithospheric structures control arc-continent collisional heterogeneity

Abstract From west to east along the Sunda-Banda arc, convergence of the Indo-Australian plate transitions from subduction of oceanic lithosphere to arc-continent collision. This region of eastern Indonesia and Timor-Leste provides an opportunity for unraveling the processes that occur during collision between a continent and a volcanic arc, and it can be viewed as the temporal transition of this process along strike. We collected a range of complementary geological and geophysical data to place constraints on the geometry and history of arc-continent collision. Utilizing ∼4 yr of new broadband seismic data, we imaged the structure of the crust through the uppermost mantle. Ambient noise tomography shows velocity anomalies along strike and across the arc that are attributed to the inherited structure of the incoming and colliding Australian plate. The pattern of anomalies at depth resembles the system of salients and embayments that is present offshore western Australia, which formed during rifting of east Gondwana. Previously identified changes in geochemistry of volcanics from Pb isotope anomalies from the inner arc islands correlate with newly identified velocity structures representing the underthrusted and subducted Indo-Australian plate. Reconstruction of uplift from river profiles from the outer arc islands suggests rapid uplift at the ends of the islands of Timor and western Sumba, which coincide with the edges of the volcanic-margin protrusions as inferred from the tomography. These findings suggest that the tectonic evolution of this region is defined by inherited structure of the Gondwana rifted continental margin of the incoming plate. Therefore, the initial template of plate structure controls orogenesis.

Generation and structural modification of the giant Kengdenongshe VMS-type Au-Ag-Pb-Zn polymetallic deposit in the East Kunlun Orogen, East Tethys: Constraints from geology, fluid inclusions, noble gas and stable isotopes

• There is mineralization zoning of Pb + Zn and Au + Ag in spatial distribution. • The Kengdenongshe polymetallic ore deposit attributes to the Au-related VMS-type. • Initial mineralization is associated with submarine volcanism in back-arc basin. • The ore deposit underwent structural modification during later basin closure. The Kengdenongshe giant Au-Ag-Pb-Zn polymetallic deposit is located in the East Kunlun Orogen (EKO). It contains about 42.2 t of Au, 608.6 t of Ag and 1.05 Mt of Pb and Zn with an average grade of Au 2.31 g/t, Ag 19.29 g/t and Pb + Zn 3.49 wt% (Pb: 1.23 wt%, Zn: 2.26 wt%). The NWW-trending ore bodies are predominantly hosted in Late Permian to Triassic rhyolitic tuff, which formed during Late Permian back-arc extension to Triassic arc-continental collision. The ore bodies are subdivided into Pb-Zn rich ore bodies on the top with high grades of Pb and Zn and low grades of Au and Ag, and Au-Ag rich ore bodies below with high grades of Au and Ag and low grades of Pb and Zn. The Pb-Zn rich ore bodies occur as vein, stockwork, and in breccia, and comprise quartz, pyrite, galena, sphalerite, and small amounts of chalcopyrite. The Au-Ag rich ore bodies consist of auriferous barite-sulfide-oxide veins and contain barite, pyrite (early strawberry and oolitic pyrite and later eu- to subhedral pyrite), galena, sphalerite, chalcopyrite, tetrahedrite and covellite. Gold is present as electrum, kustelite and native gold and silver is present as polybasite, pearceite, kongsbergite, and as minor native silver in microfractures in sulfides. The hydrothermal alteration minerals include, from bottom to top, quartz + barite + calcite around the Au-Ag rich orebodies, quartz + chlorite + epidote around the Pb-Zn rich orebodies, and quartz + K-feldspar within the tuff. Fluid inclusions from both the Pb-Zn rich and the Au-Ag rich orebodies consist of two phases (V–L-type) fluid inclusions of which the vapor phase has a size of 10–40 vol%. Fluid inclusions microthermometry reveal homogenization temperatures of fluid inclusions in Pb-Zn rich and Au-Ag rich ore bodies of 128–230 °C and 110–320 °C, with corresponding salinities of 0.7–9.9 and 0.2–18.3 wt% NaCl equivalent, respectively. H-O-S-Pb stable isotope and He-Ar noble gas isotope data indicate a mixed magmatic water-seawater source for both the Pb-Zn and Au-Ag rich ore bodies, and an additional meteoric water component for the Au-Ag rich ore bodies. The Pb-Zn and Au-Ag rich ore bodies share the same sulfur and lead sources, i.e. sulfur is derived from crustal magma and seawater/marine sulfate, and the lead originated from a mixed magmatic-ancient crustal sedimentary source. Collectively, the regional geology, mineralogy, alteration, and geochemistry indicate that the Kengdenongshe Au-Ag-Pb-Zn polymetallic deposit can be characterized as a VMS-type (volcanic-associated massive sulfide) deposit. Formation of the ore-hosting rhyolite tuff and mineralization are associated with Late Permian to Triassic marine volcanic exhalation. Middle to Late Triassic basin closure and arc-continent collision modified the deposit and resulted in the location inversion of the Pb-Zn and Au-Ag rich orebodies.

Reinterpretation of the northern South China Sea pre-Cenozoic basement and geodynamic implications of the South China continent: constraints from combined geological and geophysical records

The pre-Cenozoic northern South China Sea (SCS) Basin basement was supposed to exist as a complex of heterogeneous segments, divided by dozens of N–S faulting. Unfortunately, only the Hainan Island and the northeastern SCS region were modestly dated while the extensive basement remains roughly postulated by limited geophysical data. This study presents a systematic analysis including U-Pb geochronology, elemental geochemistry and petrographic identification on granite and meta-clastic borehole samples from several key areas. Constrained from gravity-magnetic joint inversion, this interpretation will be of great significance revealing the tectono-magmatic evolution along the southeastern margin of the Eurasian Plate. Beneath the thick Cenozoic sediments, the northern SCS is composed of a uniform Mesozoic basement while the Precambrian rocks are only constricted along the Red River Fault Zone. Further eastern part of the northern SCS below the Cenozoic succession was widely intruded by granites with Jurassic-to-early Cretaceous ages. Further western part, on the other hand, is represented by meta-sedimentary rocks with relatively sporadic granite complexes. To be noted, the western areas derived higher-degree and wider metamorphic zones, which is in contrast with the lower-degree and narrower metamorphic belt developed in the eastern region. Drastic collisions between the Indochina Block and South China continent took place since at least late Triassic, resulting in large-scale suturing and deformation zones. At the westernmost part of the northern SCS, the intracontinental amalgamation with closure of the Meso-Tethys has caused fairly stronger and broader metamorphism. One metamorphic biotite granite is located on the suturing belt and yields a Precambrian U-Pb age. It likely represents the relict from the ancient Gondwana supercontinent or its fringes. Arc-continental collision between the Paleo-Pacific and the southeast China Block, on the other hand, results in a relatively narrow NE-SW trending metamorphic belt during the late Mesozoic. Within the overall geological setting, the Cenozoic SCS oceanic basin was subsequently generated from a series of rifting and faulting processes along the collisional-accretionary continental margin.

Geochronological Framework of Paleoproterozoic Intrusive Rocks and Its Constraints on Tectonic Evolution of the Liao-Ji Belt, Sino-Korean Craton

The Liao-Ji belt (LJB) is one of the Paleoproterozoic tectonic belts located in the North China Craton. A large number of Paleoproterozoic meta-volcanic-sedimentary rock and intrusive rocks are preserved in the LJB, which provide reliable carriers for the study of the Paleoproterozoic tectonic evolution of the North China Craton. The Paleoproterozoic intrusive rock in the LJB can be divided into the following seven types: syenogranite, quartz diorite, porphyry granite, migmatitic granite, syenite, metamorphic plutonic rock, and granitic pegmatite and metagabbro (metamorphic diabase). Zircon U-Pb dating of 15 samples from intrusive rocks was carried out in this study. The chronology framework of the Paleoproterozoic intrusive rock in the LJB was established, and the magmatism of intrusive rocks can be divided into three stages: 2 200 to 2 110, 2 010 to 1 937, 1 900 to 1 820 Ma. The chronological framework supported the evolution model of subduction accretionary arc-continent collision in the LJB effectively. Combined with previous geochemical work, it was a passive continental margin environment at approximately 2 200 Ma, and then transformed into and active continental margin. The bimodal intrusive rocks between 2 180 and 2 150 Ma indicated a back-arc tension environment which lasted until approximately 2 110 Ma with a large number of basic intrusive rocks. And then the back-arc basin began to contract and the magmatic activities were reduced, with only a small number of intrusive rock activities occurring at approximately 2 040, 2 010 and 1 937 Ma. After the orogenic activities, there was a post-orogenic extension stage from 1 900 to 1 820 Ma. Magmatic activities caused by the environmental extension started to occur more frequently and subsequently resulted in the large-scale intrusive rocks in eastern Liaoning.

On the cratonization of the Arabian-Nubian Shield: Constraints from gneissic granitoids in south Eastern Desert, Egypt

The Shaitian granite complex (SGC) spans more than 80 Ma of crustal growth in the Arabian–Nubian Shield in southeast Egypt. It is a voluminous composite intrusion (60 km2) comprising a host tonalite massif intruded by subordinate dyke-like masses of trondhjemite, granodiorite and monzogranite. The host tonalite, in turn, encloses several, fine-grained amphibolite enclaves. U-Pb zircon dating indicates a wide range of crystallization ages within the SGC (800 ± 18 Ma for tonalites; 754 ± 3.9 Ma for trondhjemite; 738 ± 3.8 Ma for granodiorite; and 717 ± 3.2 Ma for monzogranite), suggesting crystallization of independent magma pulses. The high positive εNdi (+6–+8) indicate that the melting sources were dominated by juvenile material without any significant input from older crust. Application of zircon saturation geothermometry indicates increasing temperatures during the generation of melts from 745 ± 31 °C for tonalite to 810 ± 25 °C for trondhjemite; 840 ± 10 °C for granodiorite; and 868 ± 10 °C for monzogranite. The pressure of partial melting is loosely constrained to be below the stability of residual garnet (<10 kbar) as inferred from the almost flat HREE pattern ((Gd/Lu)N = 0.9–1.1), but >3 kbar for the stability of residual amphibole as inferred from the significantly lower NbN and TaN compared with LREEN and the sub-chondrite Nb/Ta ratios exhibited by the granitic phases. The inverse relation between the generation temperatures and the ages estimates of the granitoid lithologies argue against a significant role of fractional crystallization. The major and trace element contents indicate the emplacement of the SGC within a subduction zone setting. It lacks distinctive features for melt derived from a subducted slab (e.g. high Sr/Y and high (La/Yb)N ratios), and the relatively low MgO and Ni contents in all granite phases within the SGC suggest melting within the lower crust of an island arc overlying a mantle wedge. Comparison with melts produced during melting experiments indicates an amphibolite of basaltic composition is the best candidate as source for the tonalite, trondhjemite and granodiorite magmas whereas the monzogranite magma is most consistent with fusion of a tonalite protolith. Given the overlapping Sm-Nd isotope ratios as well as several trace element ratios between monzogranite and tonalite samples, it is reasonable to suggest that the renewed basaltic underplating may have caused partial melting of tonalite and the emplacement of monzogranite melt within the SGC. The emplacement of potassic granite (monzogranite) melts subsequent to the emplacement of Na-rich granites (tonalite-trondhjemite-granodiorite) most likely suggests major crustal thickening prior arc collision and amalgamation into the over thickened proto-crust of the Arabian-Nubian shield. Eventually, after complete consolidation, the whole SGC was subjected to regional deformation, most probably during accretion to the Saharan Metacraton (arc–continent collisions) in the late Cryogenian -Ediacaran times (650–542 Ma).

ГЕОДИНАМИКА И МАГМАТИЗМ ТРАНСФОРМНЫХ ОКРАИН ТИХООКЕАНСКОГО ТИПА: ОСНОВНЫЕ ТЕОРЕТИЧЕСКИЕ АСПЕКТЫ И ДИСКРИМИНАНТНЫЕ ДИАГРАММЫ

Transform margins represent lithospheric plate boundaries with horizontal sliding of oceanic plate, which in time and space replaced the subduction related convergent margins. This happened due to: spreading ridge–trench intersection (California; Queen Charlotte–Northern Cordilleran, West of the Antarctic Peninsula, and probably the Late Miocene–Pleistocene Southernmost South America) or ridge death along continental margin (Baja California); change in the direction of oceanic plate movement (Western Aleutian–Komandorsk; Southernmost tip of the Andes); and island arc-continent collision (New Guinea Island). Post-subduction magmatism is related to a slab window that resulted either from the spreading ridge collision (subduction) with a continental margin or slab tear formation, or slab break-off after subduction cessation due to other reasons. Igneous magmatic series formed in consequence of these events show diversity of tholeiitic (sub-alkaline), alkaline or calc-alkaline, high-alumina and adakitic rocks. The comprehensive geochemical dataset (more than 2400 analyses) on igneous rocks of the model transform and convergent geodynamic settings allowed to substantiate the most informative triple diagrams for the petrogenic oxides TiO2 × 10 – Fe2O3Tot – MgO and trace elements Nb – La– Yb. Mostly approved for the rock compositions with SiO2 &lt; 63 wt. %, the new plots are capable of distinguishing igneous rocks formed above zones of subduction at an island arc and continental margin (related to convergent margins), from those formed in the tectonic setting of transform margins along continents or island arcs.

Petrography and geochemistry of intrusive magmas in Varmaqan – Sardare ghobadi in the west of Iran

The study area is a quadrilateral of 155 km2 between eastern longitude 47˚ and 40 ′ to 47˚ and 52 ′ and northern latitudes 35˚ and 00 ′ to 35˚ and 04 ′ that is located in west of Iran, north of Sonqor city and between Varmaqan and Sardare Ghobadi villages of Kermanshah province. In this range, the intrusive rocks are alkaline granite, granite, granodiorite, tonalite, quartz alkaline syenite, quartz monzonite, quartz monzodiorite, quartz diorite, alkaline syenite, monzonite, diorite, gabbro diorite, gabbro, and olivine gabbro as they were injected in the iron ores of cretaceous which has resulted in contact metamorphism and created hornfels at the site of contact. After comprehensive sampling of all required igneous rocks and according to the thesis objectives, thin sections were prepared and after petrography and some samples were selected for geochemical experiments. XRF analysis, ICP and alkaline fusion were performed on some samples. According to geochemical and petrological studies, the magmas forming these intrusive igneous rocks are from one region and because of magmatic differentiation or fractional crystallization, they from basaltic to acidic terms. Samples of this quadrilateral have a meta-alumina nature and granitoids are in the range of arc islands granites, continental arc granitoids and continental collision granitoids. The mineralogical and chemical composition of the acidic rocks in the area show that the granites in this study are type I.

Stratigraphic framework and sediment wave fields associated with canyon-levee systems in the Huatung Basin offshore Taiwan Orogen

Abstract Newly compiled bathymetry, sub-bottom profiling and seismic data are used to illustrate seafloor morphology, submarine canyon networks and stratigraphic framework in Huatung Basin (HB) and an adjacent sub-basin, named as East Luzon Arc Basin offshore east Taiwan. Based on seismic facies characteristics and isopach maps, three major stratigraphic sequences are recognized and we suggest that they could be resulted from pre- and syn-Taiwan arc-continent collision. Based on the sediment thickness and sedimentation rates, ages of the HB strata could be estimated. Pelagic sediments formed the lower HB sequence which follows the HB basement reliefs since 50–102 Ma. Then, the initiation of Taiwan orogen and Luzon Arc volcanic activities changed the deposition pattern after 5 Ma, and mass transport deposits developed in middle HB sequence. After rapid uplift of the Taiwan mountain belt since about 1.5 Ma, complex canyon-levee systems and thick strata formed the upper sequence in the northern HB. In contrast, sediment thickness is thin and no submarine canyon developed in southern HB. The seafloor in South Huatung Basin is flat, except in its eastern part where couple basement ridges appeared. In North Huatung Basin, canyon-levee complex and sediment waves are the most distinct morphological features in the drainages of two submarine canyons. We propose a three-stage process for sediment wave development: (1) sediment gravity flow can initiate a base of sediment wave; (2) wavy geometry begins to develop; (3) sediment waves continuously aggrade due to overbank sediments in the canyon-levee system. This study presents the evolution of sedimentary processes in a deep-sea basin and how the deposition patterns were affected by the convergent tectonics.

Subduction zone processes and crustal growth mechanisms at Pacific Rim convergent margins: modern and ancient analogues

AbstractContinents grow mainly through magmatism, relamination, accretionary prism development, sediment underplating, tectonic accretion of seamounts, oceanic plateaus and oceanic lithosphere, and collisions of island arcs at convergent margins. The modern Pacific–Rim subduction zone environments present a natural laboratory to examine the nature of these processes. The papers in this special issue focus on the: (1) modern and ancient accretionary margins of Japan; (2) arc–continent collision zone in the Taiwan orogenic belt; (3) accreting versus non-accreting convergent margins of the Americas; and (4) several examples of ancient convergent margins of East Asia. Subduction erosion and sediment underplating are important processes, affecting the melt evolution of arc magmas by giving them special crustal isotopic characteristics. Oblique arc–continent collisions cause strong deformation partitioning that results in orogen-parallel extension, crustal exhumation and wrench faulting in the hinterland, and thrust faulting–folding in the foreland. Trench-parallel widths of subducting slabs exert major control on slab geometries, the degree of coupling–decoupling between the lower and upper plates, and subduction velocity partitioning. An initially large width of the subducting Palaeo-Pacific Plate against East Asia caused flat subduction and resistance to slab rollback during the Triassic Period. These conditions resulted in shortening across SE China. Foundering and delamination of the flat slab during the Early Jurassic Epoch led to slab segmentation and reduced slab widths, followed by slab steepening and rollback. This pull-away tectonics induced lithospheric extension and magmatism in SE China during Late Jurassic – Cretaceous time. Melting of subducted carbonaceous sediments commonly produces networks of silicate veins in CLM that may subsequently undergo partial melting, producing ultrapotassic magmas.

The significance of Upper Jurassic felsic volcanic rocks within the incipient, intraoceanic Dras Arc, Ladakh, NW Himalaya

The Dras Arc is an island arc terrane located along the Indus Suture within the Ladakh Himalaya. To the north it is juxtaposed against the Eurasian Ladakh Batholith and to the south it is thrust over the Lamayuru Complex and Indian passive margin. Establishing the timing of inception and final collision of the Dras Arc is imperative to reconstructions of the Neotethyan Ocean and timing of arc-continent collisions, prior to the terminal India-Asia continental collision. We describe and date felsic tuffs and adakitic felsic volcanic rock interbedded within the dominantly basaltic-andesitic Dras volcanic complex. These felsic volcanic units yield Upper Jurassic zircon U–Pb ages of 160 ± 3 and 156 ± 1 Ma respectively, making these the oldest reported units within the Dras Arc. We also report zircon U–Pb geochronologic and whole rock geochemical results for the Kargil Intrusive Suite which intrudes the volcanic complex. Previous ages for the intrusives have been reproduced (102 ± 2 Ma and 101 ± 2 Ma), and a second, much younger phase (80 ± 1 Ma) has been identified as one of the youngest igneous phases within the Dras Arc. The presence of felsic, adakitic volcanism early in the evolution of the Dras Arc is consistent with the adolescent stages of island arc systems, in which dehydration melting of underplated arc or subducted oceanic crust generates small volumes of felsic magmas. Thus, the intraoceanic Dras Arc initiated in the Neotethyan Ocean during the Upper Jurassic, much earlier than previously reported, and possibly was active right up to collision during the late Palaeocene between 60 and 50 Ma. It is likely that the Dras Arc developed together with the Spongtang Ophiolite-Spong Arc complex and the intraoceanic Zedong terrane of Tibet, before first colliding and accreting onto the passive margin of India, prior to the terminal continental collision.

Paleoproterozoic TTG-like metagranites from the Dahomeyide Belt, Ghana: Constraints on the evolution of the Birimian-Eburnean Orogeny

Among many Paleoproterozoic Orogens, the Birimian-Eburnean Orogeny is of particular importance due to its vital role in connecting the African and South American blocks during the Columbia supercontinent assembly. Although, several tectonic models are proposed for the Birimian-Eburnean Orogen of West Africa, more and more studies demonstrate that the Birimian-Eburnean Orogen was dominated by arc-related accretion/collision. In this study, we present geochemical characteristics, Lu-Hf-O isotopes compositions and zircon U-Pb ages of metagranites from the Dahomeyide basement of Ghana to constrain crustal thickness during the Birimian-Eburnean Orogeny. The metagranites intrude into the dominant Dahomeyide orthogneisses but are not documented. They are mainly high-K calc-alkaline, weakly peraluminous with A/CNK of 1.0–1.07. They have very high Sr/Y and (La/Yb)N, and are strongly depleted in HREE and HFSE. They exhibit potassic TTGs-like geochemical characteristics and yield zircon U-Pb ages of 2100 ± 6–2118 ± 30 Ma. They show subchondritic Lu-Hf isotope compositions with εHf(t) values of −11.1 to −1.6, and preserve typical I-type granitoids oxygen isotope composition with zircon δ18O of 6.4–6.7‰, quartz δ18O of 9.3–9.7‰, plagioclase δ18O of 7.4–7.8‰, K-feldspar δ18O of 7.0–7.3‰, biotite δ18O of 3.5–4.3‰, and whole-rock δ18O of 8.3–8.5‰. Comparison of data for the metagranites with data previously reported for the host orthogneisses indicate that the metagranites are relatively younger but preserve Lu-Hf-O signatures similar to the orthogneisses. This suggests that both rock suites were derived from melting of common Archaean igneous rocks. However, theoretical modelling of partial melting indicates that the metagranites formed in a thickened lower crust with rutile-eclogites residue in a depth ≥50 km, whereas the host orthogneisses formed in the lower crust with garnet-amphibolite residue in a depth of ≤35 km.

Building cratonic keels in Precambrian plate tectonics.

The ancient cores of continents (cratons) are underlain by mantle keels-volumes of melt-depleted, mechanically resistant, buoyant and diamondiferous mantle up to 350 kilometres thick, which have remained isolated from the hotter and denser convecting mantle for more than two billion years. Mantle keels formed only in the Early Earth (approximately 1.5 to 3.5 billion years ago in the Precambrian eon); they have no modern analogues1-4. Many keels show layering in terms of degree of melt depletion5-7. The origin of such layered lithosphere remains unknown and may be indicative of a global tectonics mode (plate rather than plume tectonics) operating in the Early Earth. Here we investigate the possible origin of mantle keels using models of oceanic subduction followed by arc-continent collision at increased mantle temperatures (150-250 degrees Celsius higher than the present-day values). We demonstrate that after Archaean plate tectonics began, the hot, ductile, positively buoyant, melt-depleted sublithospheric mantle layer located under subducting oceanic plates was unable to subduct together with the slab. The moving slab left behind craton-scale emplacements of viscous protokeel beneath adjacent continental domains. Estimates of the thickness of this sublithospheric depleted mantle show that this mechanism was efficient at the time of the major statistical maxima of cratonic lithosphere ages. Subsequent conductive cooling of these protokeels would produce mantle keels with their low modern temperatures, which are suitable for diamond formation. Precambrian subduction of oceanic plates with highly depleted mantle is thus a prerequisite for the formation of thick layered lithosphere under the continents, which permitted their longevity and survival in subsequent plate tectonic processes.

U–Pb geochronology, Nd–Hf isotopes, and geochemistry of Rhyacian granitoids from the Paleoproterozoic Lourenço domain (Brazil), southeastern Guiana Shield

The southern sector of the Paleoproterozoic Lourenço domain in the central-eastern region of the Amapá state, northern Brazil, consists mainly of Rhyacian granite-greenstone terranes with some Archean fragments. It is located at the transition with the Archean Amapá block, a Meso-Neoarchean continental landmass (2.85–2.60 Ga), strongly reworked during the Transamazonian orogeny (2.26–1.95 Ga). Geochemical and petrographic data together with combined zircon U–Pb and Lu–Hf (LA-ICP-MS) as well as whole-rock Sm–Nd (TIMS) dating, establish the chronology and petrogenesis of the granitoids and record the crustal growth and reworking at the transition between the Archean and Paleoproterozoic domains in the southeastern Guiana Shield. Two episodes, dated at ~2.18–2.16 Ga and 2.14–2.13 Ga, respectively, include medium-to high-K calc-alkaline and metaluminous to peraluminous rocks of the Flexal and Papa-Vento intrusive suites, emplaced in a magmatic arc environment. A third episode, dated at ~2.08 Ga, encompasses mostly peraluminous granites with a medium-to high-K calc-alkaline signature, of the syn-to late-collisional Vila Bom Jesus Suite. The zircon ƐHf (−3.5 to −4.3) and whole-rock ƐNd (−2.7 to −4.3) signatures with respective Hf-TDMC (2.9–3.1 Ga) and Nd-TDM (2.6–2.7 Ga) model ages indicate a mixture of Rhyacian juvenile material with a Mesoarchean crustal component for the magma sources of the Flexal and Papa-Vento intrusive suites. The geochemical characteristics, zircon ƐHf (−6.6 to −12.5) and whole rock ƐNd (−3.7 to −5.4) signatures, and Hf-TDMC (2.9–3.6 Ga) and Nd-TDM (2.7–2.9 Ga) model ages, together with the presence of Rhyacian (2.19 Ga) and Mesoarchean (2.82 Ga) inherited zircon crystals in the Vila Bom Jesus Suite, indicate dominant crustal reworking of Rhyacian magmatic arc rocks and Archean rocks from the adjacent Amapá Block during the arc-continent collision. The geochemical and isotopic signatures of the magmatic arc granitoids (Flexal and Papa-Vento intrusive suites) may be attributed to the incorporation in the juvenile magmatic sources of Archean continental sediments or hidden crustal fragments from the Amapá Block during the subduction processes. Geochemical affinities with continental magmatic arc and subchrondritic Hf–Nd isotopic signatures are consistent with a scenario of a long-lived Mesorhyacian (~2.20–2.13 Ga), Cordilleran-type magmatic arc setting, which evolved to a collisional setting with the amalgamation of the Archean Amapá block and the Rhyacian Lourenço domain during the Neorhyacian (2.11–2.08 Ga).

Early Cretaceous volcanic rocks in Yunzhug area, central Tibet, China, associated with arc–continent collision in the Tibetan Plateau?

The Lhasa-Qiangtang collision prior to the Cenozoic collision of India and Asia is a key process for the early stage of tectonic history of the Tibetan Plateau; however, when and how the Lhasa-Qiangtang collision is less well known. In this paper, we present an analysis of petrological, geochemical, Sr–Nd–Hf isotopic data, and zircon LA-ICP-MS UPb data for the volcanic rocks from the Yunzhug area, northern Lhasa. Our new data reveal an important, previously unrecognized stage of tectono-magmatic event in northern Lhasa subterrane. Zircons from the andesites and rhyolites yield concordant ages of 120.24 ± 0.79 Ma (mean square of weighted deviates (MSWD) = 0.35) and 120.1 ± 1.1 Ma (MSWD = 1.18), respectively, which are younger than the age of Jurassic intra-oceanic magmatism and older than the 112–110 Ma Lhasa-Qiangtang collision. The Yunzhug volcanic rocks are composed of andesites and A-type rhyolites of high-K affinity. The andesites originated mainly from a mixed mantle source with variable degrees of contribution of continental crust, and the rhyolites were derived mainly from melting of lower continental crust. Addition of subducted flysch or oceanic sediments resulted in the high-K affinity. Fractional crystallization of plagioclase, K-feldspar, amphibole, and minor microlitic apatite and FeTi oxides minerals in the shallow level is mainly responsible for variable major and trace element contents of the andesites and rhyolites. Our results show that the early Cretaceous Yunzhug volcanic rocks can represent the early stage magmatism recording the entrance of subducted continental crust into the subduction zone, agreement with processes of arc-continent collision from intra-oceanic subduction to continental collision. The presence of oceanic sediments or monazite residual in magma source under the arc-continent zone can cause LREE enrichment to exceed the continental crust, making arc magmatic compositions close to average continental values.

Magmatic gap associated with stepwise arc‐continent collision during the subduction of Banggong‐Nujiang Tethys Ocean: Evidence from Late Jurassic bimodal magmas

The magmatic suites in the southern margin of South Qiangtang Terrane (SQT) provide important clues to the process of northward subduction of the Banggong‐Nujiang Tethys Ocean (BNTO) and its geodynamic evolution. Previous studies identified a magmatic gap from Late Jurassic to Early Cretaceous (ca. 150–130 Ma) in the southern margin of SQT. Although the gap is generally correlated with the process related to BNTO subduction, the previous models on this remain equivocal. In this study, we investigated a suite of bimodal magmatic rocks newly recognized in the Rigenco area of SQT, including gabbros and granodiorites which were dated at 151.7 ± 3.8 Ma and 150.0 ± 2.0 Ma respectively. Based on geochemical and isotopic analyses, it is inferred that the gabbroic magmas were derived from a sub‐arc mantle source enriched by metasomatism of voluminous slab‐derived and crustal compositions, and that the granodiorites were derived from sub‐arc juvenile lower crust. Combining the petrogenesis of gabbros and granodiorites, it is proposed that the bimodal magmas were produced under a tectonic setting of general compression with local extension. A detailed evaluation of the tectonic setting of the magmatic bimodal suite suggests crustal thickening at ca. 150 Ma and that the BNTO involved multiple arcs and sub‐basins. A new model of arc‐SQT collision is proposed to account for the magmatic gap which commenced at ca. 150 Ma. The model of arc–continent collision in this case can well explain the present distribution and occurrence of magmatic rocks of late Jurassic age in the southern margin of the SQT, especially the magmatic gap. Our discovery of Late Jurassic bimodal rocks provides a critical evidence for step‐wise arc–continent collision during BNTO subduction, and contributes to the understanding of the magmatic gap in SQT and the evolution of BNTO.

Lithospheric mantle anisotropy from local events beneath the Sunda–Banda arc transition and its geodynamic implications

Shear wave splitting analysis to characterise lithospheric mantle anisotropy has been performed to provide better knowledge about lithospheric deformation and mantle flow beneath the Sunda–Banda arc transition, Indonesia. The tectonic setting of the study area is very complex characterised by the transition from subduction along Sunda arc to collision in Banda arc. The splitting measurements show lateral and vertical variation in the fast directions of the S-waves in this region. When the splitting results are analysed through 2D delay-time tomography and spatial averaging, systematic patterns in delay times and fast polarisation become more visible. In the subduction domain, the spatial averages of fast directions are dominated by two distinct fast polarisations: perpendicular and parallel to the plate motion for shallow and deep events, respectively. The results suggest that anisotropy in this area is not only controlled by anisotropic source related to the simple mantle flow model, but also by anisotropic fabric in the mantle deformed under influence of high stresses, high water contents and low temperatures. In addition, there might also be contribution from the anisotropic body in the upper layer. In the collision domain, spatially averaged fast directions show mostly perpendicular to the plate motion for all deep levels. For shallow level in this region, this trend is mainly governed by the lithospheric deformation process due to the continent-arc collision as also shown by delay time tomographic inversion. For deeper part of the region, the result of tomographic inversion and spatial averaging reveals a high anisotropy followed by rotational pattern of fast directions in the north of Timor. We suggest that this pattern might be related to the induced mantle flow due to lateral tearing of the slab.

Accretion, Soft and Hard Collision: Similarities, Differences and an Application from the Newfoundland Appalachian Orogen

We argue there is no distinction between accretion and collision as a process, except when accretion is used in the sense of incorporating small bodies of sedimentary and/or volcanic rocks into an accretionary wedge by off-scraping or underplating. There is also a distinction when these terms are used in classifying mountain belts into accretionary and collisional orogens, although such classifications are commonly based on a qualitative assessment of the scale and nature of the accreted terranes and continents involved in formation of mountain belts. Soft collisions occur when contractional deformation and associated metamorphism are principally concentrated in rocks of the leading edge of the partially pulled-down buoyant plate and the upper plate forearc terrane. Several young arc-continent collisions show evidence for partial or wholesale subduction of the forearc such that the arc is structurally juxtaposed directly against lower plate rocks. This process may explain the poor preservation of forearcs in the geological record. Soft collisions generally change into hard collisions over time, except if the collision is rapidly followed by formation of a new subduction zone due to step-back or polarity reversal. Thickening and metamorphism of the arc's suprastructure and retro-arc part of upper plate due to contractional deformation and burial are the characteristics of a hard collision or an advancing Andean-type margin. Strong rheological coupling of the converging plates and lower and upper crust in the down-going continental margin promotes a hard collision. Application of the soft–hard terminology supports a structural juxtaposition of the Taconic soft collision recorded in the Humber margin of western Newfoundland with a hard collision recorded in the adjacent Dashwoods block. It is postulated that Dashwoods was translated dextrally along the Cabot-Baie Verte fault system from a position to the north of Newfoundland where the Notre Dame arc collided ca. 10 m.y. earlier with a wide promontory in a hyperextended segment of the Laurentian margin.

Emergence of the Southeast Asian islands as a driver for Neogene cooling

Steep topography, a tropical climate, and mafic lithologies contribute to efficient chemical weathering and carbon sequestration in the Southeast Asian islands. Ongoing arc-continent collision between the Sunda-Banda arc system and Australia has increased the area of subaerially exposed land in the region since the mid-Miocene. Concurrently, Earth's climate has cooled since the Miocene Climatic Optimum, leading to growth of the Antarctic ice sheet and the onset of Northern Hemisphere glaciation. We seek to evaluate the hypothesis that the emergence of the Southeast Asian islands played a significant role in driving this cooling trend through increasing global weatherability. To do so, we have compiled paleoshoreline data and incorporated them into GEOCLIM, which couples a global climate model to a silicate weathering model with spatially resolved lithology. We find that without the increase in area of the Southeast Asian islands over the Neogene, atmospheric pCO2 would have been significantly higher than preindustrial values, remaining above the levels necessary for initiating Northern Hemisphere ice sheets.

Neoarchean to Palaeoproterozoic tectonic evolution of the Trans‐North China Orogen, North China Craton: Evidence from zircon U–Pb geochronology, Lu–Hf isotopes, and geochemistry of the Zanhuang Complex

Research on the Zanhuang Complex has been one of the hotspots in the Neoarchean–Palaeoproterozoic tectonic evolution of the Trans‐North China Orogen (TNCO) in the North China Craton (NCC). Here, we present data from petrology, geochemistry, zircon U–Pb geochronology, and Lu–Hf isotopes on a suite of granite, diorite, TTG gneiss, and amphibolite in the Zanhuang Complex to characterize these rocks and understand their genesis. Zircon U–Pb data yield magmatic ages of 2,607 Ma for granite, 2,599 Ma for diorite and 2,561 Ma for TTG gneiss, with subsequent thermal event ages of 2,530 and 2,471 Ma. Lu–Hf isotopes show εHf(t) values ranging from 0.04 to 7.11 and crustal model ages of 2,700–3,100 Ma for the diorite, indicating a Meso‐ to Neoarchean mantle source with minor input of crustal components. The granite and TTG gneiss have the characteristics of volcanic‐arc granite, with low Sr/Y and (La/Yb)N, high Cr and Ni contents, positive Pb and K anomalies and negative Nb, Ta, P, and Ti anomalies. The amphibolite and diorite are similar to island arc basalt, with low K2O, Sr/Y, (La/Yb)N, and high FeOT, positive Pb anomaly, negative Nb, Ta, and Ti anomalies and absence of obvious Ce anomaly. These signatures indicate that these rocks were likely formed in a subduction setting. We have proposed a model of subduction–collision process in the TNCO, including subduction at ca. 2.70–2.55 Ga, arc–continent collision at ca. 2.55–2.45 Ga, extension and rifting at ca. 2.30–2.05 Ga, and continent–continent collision at ca. 1.90–1.75 Ga. Our results are consistent with the previous models on the tectonic evolution in the NCC and provide further insights into the Neoarchean–Palaeoproterozoic tectonic history of the craton.