Fraukje M. Brouwer

Late Neoproterozoic proto-arc ocean crust in the Dariv Range, Western Mongolia: a supra-subduction zone end-member ophiolite

An unusual late Neoproterozoic (c. 572 Ma) ophiolite is exposed in the Dariv Range (western Mongolia), which contains intermediate to acidic lavas and sheeted dykes, and an igneous layered complex consisting of gabbro–norites, websterites, orthopyroxenites and dunites underlain by serpentinized mantle harzburgites. Based on the compositions of the crustal units and the crystallization sequences in the mafic and ultramafic cumulates we conclude that the entire oceanic crust, including the cumulates, was made from arc magmas with boninitic characteristics. The Dariv rocks bear a strong resemblance to rocks recovered from the modern Izu–Bonin–Mariana fore-arc, a fragment of proto-arc oceanic basement, and we propose that the Dariv Ophiolite originated in a similar tectonic setting. A metamorphic complex consisting of amphibolite- to granulite-facies metasedimentary and meta-igneous rocks was thrust over the ophiolite. This metamorphic complex probably represents a Cambrian arc. Thrusting started before 514.7 ± 7.6 Ma as constrained by new sensitive high-resolution ion microprobe U–Pb zircon analyses from a syn- to post-tectonic diorite. The Dariv Ophiolite is a type-example of a proto-arc ophiolite, a special class of supra-subduction zone ophiolites.

Late-orogenic heating during exhumation: Alpine PTt trajectories and thermomechanical models

Abstract During the Alpine orogeny, the Penninic zone of the Alps was affected by Eoalpine high-pressure metamorphism. In the central and western Alps, this was followed by Lepontine medium-pressure, high-temperature metamorphism during exhumation. We compare the pressure–temperature–time (PTt) trajectories established in two key areas in the central and western Alps with 2-D numerical models of two possible causes of Lepontine metamorphism: (1) detachment or breakoff of a subducting slab, and (2) the presence of a wedge of accreted radiogenic material. Numerical models show that both mechanisms are capable of producing significant heating during orogeny. Heating by slab detachment is fast and transient (more than 100°C in up to 10 million years, depending on the location), whereas radiogenic heating requires time spans of the order of tens of millions of years and cessation of the subduction process. The combination of PTt trajectories and synthetic PT paths deduced from our thermomechanical modelling results suggests that the metamorphism observed in the central Alps has not been caused by radiogenic heating alone. Slab breakoff, on the other hand, seems a viable mechanism to account for the documented rise in metamorphic temperatures during exhumation. In view of the time constraints posed by the geological data, and acknowledging the effects of large-scale block rotations and out-of-section transport, slab detachment is also a more likely mechanism to have provided sufficient heat to cause re-heating in the western Alps.

Tectonic evolution and paleogeography of the Kırşehir Block and the Central Anatolian Ophiolites, Turkey

In Central and Western Anatolia two continent-derived massifs simultaneously underthrusted an oceanic lithosphere in the Cretaceous and ended up with very contrasting metamorphic grades: high pressure, low temperature in the Tavsanli zone and the low pressure, high temperature in the Kirsehir Block. To assess why, we reconstruct the Cretaceous paleogeography and plate configuration of Central Anatolia using structural, metamorphic, and geochronological constraints and Africa-Europe plate reconstructions. We review and provide new 40Ar/39Ar and U/Pb ages from Central Anatolian metamorphic and magmatic rocks and ophiolites and show new paleomagnetic data on the paleo-ridge orientation in a Central Anatolian Ophiolite. Intraoceanic subduction that formed within the Neotethys around 100–90 Ma along connected N-S and E-W striking segments was followed by overriding oceanic plate extension. Already during suprasubduction zone ocean spreading, continental subduction started. We show that the complex geology of central and southern Turkey can at first order be explained by a foreland-propagating thrusting of upper crustal nappes derived from a downgoing, dominantly continental lithosphere: the Kirsehir Block and Tavsanli zone accreted around 85 Ma, the Afyon zone around 65 Ma, and Taurides accretion continued until after the middle Eocene. We find no argument for Late Cretaceous subduction initiation within a conceptual “Inner Tauride Ocean” between the Kirsehir Block and the Afyon zone as widely inferred. We propose that the major contrast in metamorphic grade between the Kirsehir Block and the Tavsanli zone primarily results from a major contrast in subduction obliquity and the associated burial rates, higher temperature being reached upon higher subduction obliquity.

Dynamics of intraoceanic subduction initiation : 2. Suprasubduction zone ophiolite formation and metamorphic sole exhumation in context of absolute plate motions

Analyzing subduction initiation is key for understanding the coupling between plate tectonics and the underlying mantle. Here we focus on suprasubduction zone (SSZ) ophiolites and how their formation links to intraoceanic subduction initiation in an absolute plate motion frame. SSZ ophiolites form the majority of exposed oceanic lithosphere fragments and are widely recognized to have formed during intraoceanic subduction initiation. Structural, petrological, geochemical, and plate kinematic constraints on their kinematic evolution show that SSZ crust forms at fore-arc spreading centers at the expense of a mantle wedge, thereby flattening the nascent slab. This leads to the typical inverted pressure gradients found in metamorphic soles that form at the subduction plate contact below and during SSZ crust crystallization. Former spreading centers are preserved in forearcs when subduction initiates along transform faults or off-ridge oceanic detachments. We show how these are reactivated when subduction initiates in the absolute plate motion direction of the inverting weakness zone. Upon inception of slab pull due to, e.g., eclogitization, the sole is separated from the slab, remains welded to the thinned overriding plate lithosphere, and can become intruded by mafic dikes upon asthenospheric influx into the mantle wedge. We propound that most ophiolites thus formed under special geodynamic circumstances and may not be representative of normal oceanic crust. Our study highlights how far-field geodynamic processes and absolute plate motions may force intraoceanic subduction initiation as key toward advancing our understanding of the entire plate tectonic cycle.

Late Devonian to early Carboniferous arc-related magmatism in the Baolidao arc, Inner Mongolia, China: Significance for southward accretion of the eastern Central Asian orogenic belt

The Central Asian orogenic belt, formed in response to consumption of the Paleo–Asian Ocean, is one of the largest and most complex accretionary collages in the world and was responsible for considerable Phanerozoic juvenile crustal growth in Central Asia. The timing of subduction-accretion processes and closure of the Paleo–Asian Ocean is controversial. The Xilingol complex, composed of deformed quartzofeldspathic rocks and lenticular or quasi-lamellar amphibolites, is located on the northern section of the eastern Central Asian orogenic belt in Inner Mongolia, China. In this paper, we present a systematic study of the petrology, whole-rock geochemistry, and geochronology of an amphibolite and an epidote amphibolite from the complex. The protolith of the amphibolite is a gabbro or gabbroic diorite with a laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) zircon U-Pb age of 382 ± 4 Ma and uniform eHf(t) values (–1.06 to +1.25). The protolith of the epidote amphibolite is a quartz diorite with a LA-ICP-MS zircon U-Pb age of 327 ± 5 Ma and uniform positive eHf(t) values (+0.78 to +4.11). The primitive magma of the Devonian gabbroic dike was generated by partial melting of a spinel lherzolite lithospheric mantle that was modified by fluids and melts from subducted slab components. A newly enriched lithospheric mantle is a possible source region for the Devonian mafic rocks. Fractionation of olivine and hornblende played a dominant role in magma differentiation with little or no crustal contamination. Amphibolite-facies metamorphism affected the Devonian gabbroic rocks at 321.6 ± 3.1 Ma, and the quartz diorite underwent epidote amphibolite-facies metamorphism at 279.4 ± 5.3 Ma, based on hornblende 40Ar/39Ar dating. The Devonian to Carboniferous intrusive rocks in the eastern Central Asian orogenic belt likely formed during the collapse of a mature arc at the southern margin of the South Mongolian microcontinent. Combining our results with previous data, we identify an initial phase of postcollisional extension (382–340 Ma) that occurred after earlier compression related to intra-oceanic subduction (484–469 Ma), ridge subduction (440–434 Ma), and arc-continent collision (427–383 Ma). We also constrain increasing extension accompanying extensive collapse of the mature arc (340–309 Ma), northward subduction of the forearc oceanic crust (322–274 Ma) coeval with development of the Hegenshan back-arc basin (354–269 Ma), and final collision (246–228 Ma). The presence of an accretionary belt along the southern margin of the South Mongolian microcontinent reflects the importance of continental growth by accretion of an arc chain during the Late Cambrian–Middle Triassic in the eastern Central Asian orogenic belt.

Coherence of the Dabie Shan UHPM terrane investigated by Lu-Hf and 40 Ar/ 39 Ar dating of eclogites

Publisher Summary The Central China Orogenic Belt (CCOB) is the largest known ultrahigh-pressure metamorphic (UHPM) belt. It extends from Sulu in Eastern China, via the Dabie, Qinling, and North Qaidam mountains to Altyn Tagh some 3000 km to the west. Since the discovery of coesite and diamond-bearing metamorphic rocks in Dabie Shan, the belt has been studied intensively. Some of the results have led to controversy regarding the number of (U)HP metamorphic events and their absolute and relative ages. The prominence of Dabie Shan as part of the largest UHPM belt is reflected by a wealth of published work since the discovery of coesite over 20 years ago. Despite two decades of efforts to decipher its geologic secrets, interpretation of the many reported isotopic ages of the Dabie Shan terrane that aimed to address its metamorphic history is still hampered by the absence of robust thermobarometric data for the same samples. Pre-Triassic metamorphism cannot be traced by Lu–Hf Grt–Cpx geochronology in the main volume of Dabie Shan, which was subjected to Triassic (U)HP metamorphism. The Triassic event erased previous signatures of some, but not all isotopic systems, as is evident from zircon U-Pb ages.

Occurrence of Excess 40Ar in Amphibole: Implications of 40Ar/39Ar Dating by Laser Stepwise Heating and in vacuo Crushing

The joint methods of 40Ar/39Ar laser stepwise heating and in vacuo crushing have been applied to date amphiboles from the North Qaidam ultra-high pressure metamorphic amphibolites. Two amphibole samples analyzed by laser heating yielded saddle-shaped age spectra with total gas ages of 574.5±2.5 and 562.5±2.5 Ma. These ages are much older than the reported zircon U-Pb ages (∼495 Ma) from Yuka eclogite, indicating the presence of excess 40Ar. In order to decipher the occurrence of excess 40Ar and constrain the age of amphibolite-facies retrogression, two duplicate amphibole samples were further employed for 40Ar/39Ar in vacuo crushing analyses. Both samples exhibit similar monotonically declining release spectra, which are characterized by rapid decline of anomalously old apparent ages in the early steps. The data of the late steps yielded concordant apparent ages with plateau ages of 460.9±1.2 and 459.6±1.8 Ma. We interpret that gases released in the early steps derive from the significant excess 40Ar containing secondary fluid inclusions (SFIs) due to their distribution characteristics along cracks leading to be easily extracted, whereas those released in the later steps represent the contribution of the small primary fluid inclusions (PFIs).

Metamorphic P-T path differences between the two UHP terranes of Sulu orogen, Eastern China: petrologic comparison between eclogites from Donghai and Rongcheng.

The Sulu Orogen constitutes the eastern part of the Sulu-Dabie Orogen formed by Triassic collision between the Sino-Korean and Yangtze plates. An HP Slice I and two UHP slices II and III with contrasting subduction and exhumation histories within the Sulu Orogen were postulated. This study presents the metamorphic P-T paths of eclogites from the two UHP belts constructed by petrography, mineral chemistry and Perple_X P-T pseudosection modeling in the MnC(K)NFMASHO system. Eclogites from Slice III mainly consist of omphacite, garnet and quartz, with minor rutile, ilmenite, amphibole and phengite. Eclogites from Slice II show a porphyroblastic texture with epidote porphyroblasts and garnet, omphacite, phengite, quartz and rutile in matrix. Pseudosection modeling reveals that eclogites from Slice II witness a peak metamorphism of eclogite-facies under conditions of 3.1–3.3 GPa and 660–690 ºC, and a retrograde cooling decompression process. The eclogites from Slice III record a heating decompressive P-T path with a peak-P stage of 3.2 GPa and 840 ºC and a peak-T stage of 2.4 GPa and 950 ºC, suggesting an apparent granulite-facies metamorphism overprint during exhumation. Both eclogites recorded clockwise P-T paths with peak P-T conditions suggesting a subduction beneath the Sino-Korean Plate to ~100–105 km depth. Combined with tectonic scenarios from previous studies, it is concluded that the two UHP crustal slices in the Sulu terrane have a similar geodynamic evolution, but the UHP rocks in Slice II exhumed after the eclogitic peak-pressure conditions earlier than that of Slice III. The existence of Slice II diminished the buoyancy force on Slice III, resulting in a granulite-facies overprint on Slice III. The Sulu orogenic belt is made up of different crustal slices that underwent different subduction and exhumation histories, rather than a single unit.

Petrology and Metamorphic P-T Paths of Metamorphic Zones in the Huangyuan Group, Central Qilian Block, NW China

The Central Qilian Block is a Precambrian block in the Qilian Orogen, which has long drawn international attention for the study of orogeny and continental dynamics. The Huangyuan Group in the Datong area is one of the Precambrian metamorphic basement units in the Central Qilian Block and reflects metamorphism in the Barrovian garnet zone and sillimanite zone from south to north. Based on detailed fieldwork, this study presents a systematic study of petrography, mineral chemistry and phase equilibria of schists and gneisses from the two metamorphic zones. The garnet metamorphic zone is composed of micaschist, garnet-bearing micaschist and felsic leptynite, with interlayered plagioclase amphibolite. The sillimanite metamorphic zone consists of garnet-bearing biotite micaschist, sillimanite-bearing biotite-plagioclase gneiss and felsic leptynite. Garnet from the garnet metamorphic zone shows growth zoning with increasing almandine and pyrope and decreasing spessartine from core to rim. Garnet from the sillimanite metamorphic zone is almost homogeneous. Towards the outer rim, the contents of almandine and pyrope slightly decrease and grossular slightly increase. Biotite in both metamorphic zones is ferro-biotite. Plagioclase is oligoclase in garnet metamorphic zone and andesine in sillimanite metamorphic zone. Phase equilibrium modeling of a sample from garnet metamorphic zone resulted in a clockwise P-T path with a prograde stage (4.5–5.0 kbar, 520–530 °C), a peak P stage (9.8–10.2 kbar, 560–570 °C), a stage of thermal relaxation (8.0–8.5 kbar, 580–590 °C) and finally a retrograde stage (6.8–7.0 kbar, 560–580 °C). Thermodynamic modeling of a sample from the sillimanite metamorphic zone indicates a prograde stage (5.5–6.0 kbar, 540–550 °C) and a peak stage (7.8–8.5 kbar, 660–690 °C). The results indicate that the Huangyuan Group experienced medium-pressure amphibolite-facies metamorphism, which resulted from continental-continental collision between the Qaidam Block and the Central Qilian Block.