Petrogenesis and Lu–Hf Dating of (Ultra)Mafic Rocks from the Kutná Hora Crystalline Complex: Implications for the Devonian Evolution of the Bohemian Massif

Abstract The Lu–Hf isotope system and Sr–Nd–Hf–Os isotope systematics of mantle rocks are capable of unravelling the early processes in collision belts, especially in a hot subduction context where the Sm–Nd and U–Pb systems in crustal rocks are prone to resetting owing to high temperatures and interaction with melts during exhumation. To improve models of the Devonian–Carboniferous evolution of the Bohemian Massif, we investigated in detail mafic and ultramafic rocks (eclogite, pyroxenite, and peridotite) from the ultrahigh-pressure and ultrahigh-temperature Kutná Hora Crystalline Complex (KHCC: Úhrov, Bečváry, Doubrava, and Spačice localities). Petrography, multiphase solid inclusions, major and trace element compositions of rocks and minerals, and radiogenic isotopic data document contrasting sources and protoliths as well as effects of subduction-related processes for these rocks. The Úhrov peridotite has a depleted composition corresponding to the suboceanic asthenospheric mantle, whereas Bečváry and Doubrava peridotites represent lithospheric mantle that underwent melt refertilization by basaltic and SiO2-undersaturated melts, respectively. Multiphase solid inclusions enclosed in garnet from Úhrov and Bečváry peridotites represent trapped H2O ± CO2-bearing metasomatizing agents and Fe–Ti-rich melts. The KHCC eclogites either formed by high-pressure crystal accumulation from mantle-derived basaltic melts (Úhrov) or represent a fragment of mid-ocean ridge basalt-like gabbroic cumulate (Spačice) and crustal-derived material (Doubrava) both metamorphosed at high P–T conditions. The Lu–Hf age of 395 ± 23 Ma obtained for the Úhrov peridotite reflects garnet growth related to burial of the asthenospheric mantle during subduction of the oceanic slab. By contrast, Spačice and Doubrava eclogites yield younger Lu–Hf ages of ∼350 and 330 Ma, respectively, representing mixed ages as demonstrated by the strong granulite-facies overprint and trace element zoning in garnet grains. We propose a refined model for the Early Variscan evolution of the Bohemian Massif starting with the subduction of the oceanic crust (Saxothuringian ocean) and associated oceanic asthenospheric mantle (Úhrov) beneath the Teplá–Barrandian at ≥380 Ma, which was responsible for melt refertilization of the associated mantle wedge (Bečváry, Doubrava). This was followed by continental subduction (∼370–360 Ma?) accompanied by the oceanic slab break-off and incorporation of the upwelling asthenospheric mantle into the Moldanubian lithospheric mantle and subsequent coeval exhumation of mantle and crustal rocks at ∼350–330 Ma.

Download Full-text

Petrogenesis and Lu–Hf dating of (ultra)mafic rocks from the Kutná Hora Crystalline Complex: implications for the Devonian evolution of the Bohemian Massif

10.5194/egusphere-egu2020-7983 ◽

2020 ◽

Author(s):

Jana Kotková ◽

Lukáš Ackerman ◽

Renata Čopjaková ◽

Jiří Sláma ◽

Jakub Trubač ◽

...

Keyword(s):

Trace Element ◽

Oceanic Crust ◽

Bohemian Massif ◽

Lithospheric Mantle ◽

Mantle Wedge ◽

Mantle Melting ◽

Crystalline Complex ◽

Crustal Rocks ◽

Asthenospheric Mantle ◽

Basaltic Melts

Orogenic garnet peridotites with associated garnet pyroxenites and eclogites in the (U)HP-(U)HT terranes provide insight into mantle melting and subduction-related metamorphism in collisional orogenic belts. Here we demonstrate that they also represent unique tracers of early subduction processes in the internal part of the European Variscan Belt, where subsequent high-temperature processes affect thermochronometers in crustal rocks. Our study focused on several localities within the Kutn&#225; Hora Crystalline Complex (KHCC), a key area for the evolution of the Variscan Bohemian Massif due to its position, evidence for a deep crustal subduction (diamond in granulites) and complete geochronological record.The mantle rocks show highly variable petrographical and geochemical characteristics reflecting derivation from contrasting mantle sources which have undergone both mantle melting and enrichment due to subduction-related metasomatism.&#160; While the &#218;hrov lherzolite has trace element and Sr&#8211;Nd&#8211;Hf composition similar to depleted oceanic asthenospheric mantle, the composition of the Be&#269;v&#225;ry lherzolite reflects extensive refertilization by basaltic melts associated with Grt&#177;Cpx precipitation. Multiple solid inclusions (MSI) trapped in garnet, dominated by Ti and Fe-Ti oxides (rutile, ilmenite), represent relics of Ti-rich low-degree basaltic partial melt. Minor hornblende/phlogopite and carbonate reflect mantle metasomatism by H2O&#177;CO2-bearing fluids. Highly to mildly radiogenic Sr&#8211;Nd&#8211;Hf&#8211;Os isotopic compositions along with negative HFSE anomalies in clinopyroxene indicate only a very small contribution of recycled crustal component. The Doubrava peridotites exhibit marked petrographic variability ranging from harzburgite to composite dunite-wehrlite/olivine-bearing pyroxenite assemblage and contrasting geochemical patterns. This can be best explained by interaction between depleted protolith and SiO2-undersaturated melt with small proportion of recycled crust (~5 % when subducted oceanic crust is considered). The KHCC eclogites show diverse origins, involving products of high-pressure crystal accumulation from mantle-derived basaltic melts, or a fragment of MORB-like gabbroic cumulate and crustal-derived material both metamorphosed at HT&#8211;HP conditions.The &#218;hrov peridotite yields Lu&#8211;Hf age of 395 &#177; 23 Ma, interpreted as dating garnet growth based on detailed examination of trace element garnet zoning. By contrast, eclogites yield younger Lu&#8211;Hf ages of ~350 and 330 Ma, respectively, representing mixed ages as demonstrated by garnet trace element zoning and a strong granulite-facies overprint.We propose a refined model for Devonian&#8211;Carboniferous evolution of the Bohemian Massif, &#160;&#160;with the subduction of the oceanic crust and associated oceanic asthenospheric mantle beneath the Tepl&#225;&#8211;Barrandian at ~400 Ma related to closure of the Saxothuringian ocean between Gondwana-derived microcontinents. The overlaying lithospheric mantle wedge was refertilized by fluids/melts. Oceanic subduction passed to continental subduction of the Saxothuringian crust (~370&#8211;360 Ma?) accompanied by the break-off &#160;of the eclogitized oceanic crust facilitating incorporation of the upwelling asthenospheric mantle into the Moldanubian lithospheric mantle wedge. Subsequent collision and coeval exhumation of mantle and crustal rocks occurred at ~350&#8211;330 Ma and might be associated with mixing/mingling of crustal-derived melts and mafic lithologies producing the observed geochemical and geochronological signatures.

Download Full-text

Continental versus oceanic subduction zones

National Science Review ◽

10.1093/nsr/nww049 ◽

2016 ◽

Vol 3 (4) ◽

pp. 495-519 ◽

Cited By ~ 88

Author(s):

Yong-Fei Zheng ◽

Yi-Xiang Chen

Keyword(s):

Subduction Zones ◽

Mantle Wedge ◽

Ultrahigh Pressure ◽

Metamorphic Rocks ◽

Crustal Rocks ◽

Incompatible Elements ◽

Ultrahigh Temperature ◽

Subducting Slab ◽

Oceanic Slab ◽

Metasomatized Mantle

Abstract Subduction zones are tectonic expressions of convergent plate margins, where crustal rocks descend into and interact with the overlying mantle wedge. They are the geodynamic system that produces mafic arc volcanics above oceanic subduction zones but high- to ultrahigh-pressure metamorphic rocks in continental subduction zones. While the metamorphic rocks provide petrological records of orogenic processes when descending crustal rocks undergo dehydration and anataxis at forearc to subarc depths beneath the mantle wedge, the arc volcanics provide geochemical records of the mass transfer from the subducting slab to the mantle wedge in this period though the mantle wedge becomes partially melted at a later time. Whereas the mantle wedge overlying the subducting oceanic slab is of asthenospheric origin, that overlying the descending continental slab is of lithospheric origin, being ancient beneath cratons but juvenile beneath marginal arcs. In either case, the mantle wedge base is cooled down during the slab–wedge coupled subduction. Metamorphic dehydration is prominent during subduction of crustal rocks, giving rise to aqueous solutions that are enriched in fluid-mobile incompatible elements. Once the subducting slab is decoupled from the mantle wedge, the slab–mantle interface is heated by lateral incursion of the asthenospheric mantle to allow dehydration melting of rocks in the descending slab surface and the metasomatized mantle wedge base, respectively. Therefore, the tectonic regime of subduction zones changes in both time and space with respect to their structures, inputs, processes and products. Ophiolites record the tectonic conversion from seafloor spreading to oceanic subduction beneath continental margin, whereas ultrahigh-temperature metamorphic events mark the tectonic conversion from compression to extension in orogens.

Download Full-text

Transition from the lithospheric to asthenospheric mantle-derived magmatism in the Early Jurassic along eastern Bangong–Nujiang Suture, Tibet: Evidence for continental arc extension induced by slab rollback

Geological Society of America Bulletin ◽

10.1130/b35554.1 ◽

2020 ◽

Vol 133 (1-2) ◽

pp. 134-148

Author(s):

Wang-Chun Xu ◽

Hong-Fei Zhang ◽

Li-Ran Chen ◽

Bi-Ji Luo ◽

Liang Guo ◽

...

Keyword(s):

Mantle Source ◽

Subduction Zones ◽

Granulite Facies ◽

Early Jurassic ◽

Basement Complex ◽

Granulite Facies Metamorphism ◽

Asthenospheric Mantle ◽

Isotopic Compositions ◽

Slab Rollback ◽

Oceanic Slab

Abstract The transition of the geochemical signature in mafic rocks along the eastern Bangong–Nujiang suture in Tibet contains important information about geodynamic processes in the upper mantle. This study recognized two episodes of Early Jurassic gabbros from the Kaqiong terrane, a microblock within the Bangong–Nujiang suture zone. Early gabbros (ca. 197–191 Ma) appear as lenses in the basement complex and were overprinted by amphibolite/granulite-facies metamorphism at ca. 180 Ma. Later undeformed hornblende gabbros (ca. 177–175 Ma) occur as dikes intruding into the basement complex. The early metagabbros are characterized by arc-like geochemical features and enriched Nd-Hf isotopic compositions (whole rock ∑Nd(t) = –0.7 to +0.3; zircon ∑Hf(t) = –5.7 to –2.2), which suggests formation by partial melting of an enriched lithospheric mantle source. In contrast, the later hornblende gabbros have depleted Nd-Hf isotopic compositions (whole rock ∑Nd(t) = +6.1 to +7.1; zircon ∑Hf(t) = +10.7 to +16.8) and normal mid–oceanic–ridge basalt (N–MORB)-type rare earth element (REE) features. They also show variable enrichments of fluid mobile elements (e.g., Rb, U, Pb), indicative of the input of slab-derived fluids in their mantle source. Thus, the hornblende gabbros were most likely originated from the asthenospheric mantle metasomatized by subducted oceanic slab-derived fluids. The transition in geochemical and isotopic compositions of these mantle-derived magmas reveals a long-lasting lithosphere extension and thinning along the southern margin of the Qiangtang terrane in the Early Jurassic. Combined with geological observations, we propose that this transition has resulted from the southward rollback of the subducting Bangong–Nujiang Tethyan oceanic slab. The slab rollback could have initiated the overriding plate extension and the asthenosphere upwelling. Wider implications of this study are that an onset of slab rollback could be an important trigger for the transition of magmatic geochemistry in subduction zones.

Download Full-text

Trace-element and Nd isotopic variations in Early Proterozoic dyke swarms emplaced in the vicinity of the Kapuskasing structural zone: enriched mantle or assimilation and fractional crystallization (AFC) process?

Canadian Journal of Earth Sciences ◽

10.1139/e91-003 ◽

1991 ◽

Vol 28 (1) ◽

pp. 26-36 ◽

Cited By ~ 12

Author(s):

M. Boily ◽

J. N. Ludden

Keyword(s):

Trace Element ◽

Fractional Crystallization ◽

Lower Crust ◽

Crustal Material ◽

Early Proterozoic ◽

Crustal Rocks ◽

Asthenospheric Mantle ◽

Enriched Mantle ◽

Element Enrichment ◽

Dyke Swarms

Several Early Proterozoic Hearst–Matachewan (2.454 Ga), Kapuskasing (2.14 Ga), and Preissac (2.04 Ga) dykes were emplaced within the Archean crust surrounding the Kapuskasing structural zone (KSZ). The dykes are composed of moderately to highly fractionated tholeiitic basalts (Mg number = 24–55) that exhibit trace-element characteristics similar to those of intraplate basaltic magmas or ocean–island basalts (e.g., Zr/Nb = 6–21, Zr/Y = 2–5, high TiO2 = 0.9–3.2 wt.%, and (Fe2O3)t = 12.4–18.7 wt.%). Their initial Nd isotopic compositions display a range of depleted [Formula: see text] to enriched [Formula: see text] values that are negatively correlated with the degree of light rare-earth element enrichment. We evaluate two models for the origin of these dykes: (i) The basaltic parental magmas were derived from two distinct sources, an isotopically depleted asthenospheric mantle (εNd = +4 and La/Sm = 2.7) and an isotopically enriched lithospheric(?) mantle (εNd = −4 to−8 and La/Sm = 5.1). The magmas subsequently underwent mixing and fractionation during ascent in the mantle or the lower crust. (ii) The parental magmas originated from a homogeneous Nd isotopically depleted asthenospheric mantle but later assimilated a substantial amount of Archean crustal material upon fractionation and ascent in the lower crust. Results derived for the latter model preclude any participation of the exposed crustal rocks in the KSZ, and the assimilation and fractional crystallization (AFC) model remains a viable hypothesis only if the parental magmas assimilated an older and perhaps more isotopically enriched crust than that represented in the KSZ.

Download Full-text

Zircon U–Pb age and trace element evidence for Paleoproterozoic granulite-facies metamorphism and Archean crustal rocks in the Dabie Orogen

Lithos ◽

10.1016/j.lithos.2007.07.008 ◽

2008 ◽

Vol 101 (3-4) ◽

pp. 308-322 ◽

Cited By ~ 177

Author(s):

Yuan-Bao Wu ◽

Yong-Fei Zheng ◽

Shan Gao ◽

Wen-Fang Jiao ◽

Yong-Sheng Liu

Keyword(s):

Trace Element ◽

Granulite Facies ◽

Dabie Orogen ◽

Granulite Facies Metamorphism ◽

Crustal Rocks ◽

Facies Metamorphism

Download Full-text

Major-trace element and Sr-Nd isotope compositions of mafic dykes of the Singhbhum Craton: Insights into evolution of the lithospheric mantle

Lithos ◽

10.1016/j.lithos.2020.105959 ◽

2021 ◽

Vol 382-383 ◽

pp. 105959

Author(s):

Om Prakash Pandey ◽

Klaus Mezger ◽

Dewashish Upadhyay ◽

Debajyoti Paul ◽

Ajay Kumar Singh ◽

...

Keyword(s):

Trace Element ◽

Lithospheric Mantle ◽

Nd Isotope ◽

Mafic Dykes ◽

Singhbhum Craton

Download Full-text

Synconvergent and coherent (ultra)high-pressure crustal rock exhumation

10.5194/egusphere-egu21-11119 ◽

2021 ◽

Author(s):

Lorenzo G. Candioti ◽

Joshua D. Vaughan-Hammon ◽

Thibault Duretz ◽

Stefan M. Schmalholz

Keyword(s):

Numerical Models ◽

A Priori ◽

Western Alps ◽

Ultrahigh Pressure ◽

Plate Motion ◽

Crustal Rock ◽

Plate Boundaries ◽

Crustal Rocks ◽

Natural Data ◽

The Alps

Ultrahigh-pressure (UHP) continental crustal rocks were first discovered in the Western Alps in 1984 and have since then been observed at many convergent plate boundaries worldwide. Unveiling the processes leading to the formation and exhumation of (U)HP metamorphic crustal rocks is key to understand the geodynamic evolution of orogens such as the Alps.&#160;Previous numerical studies investigating (U)HP rock exhumation in the Alps predicted deep (>80 km) subduction of crustal rocks and rapid buoyancy-driven exhumation of mainly incoherent (U)HP units, involving significant tectonic mixing forming so-called m&#233;langes. Furthermore, these predictions often rely on excessive erosion or periods of divergent plate motion as important exhumation mechanism. Inconsistent with field observations and natural data, application of these models to the Western Alps was recently criticised.&#160;Here, we present models with continuous plate convergence, which exhibit local tectonic-driven upper plate extension enabling compressive- and buoyancy-driven exhumation of coherent (U)HP units along the subduction interface, involving feasible erosion.&#160;The two-dimensional petrological-thermo-mechanical numerical models presented here predict both subduction initiation and serpentinite channel formation without any a priori prescription of these two features. The (U)HP units are exhumed coherently, without significant internal deformation. Modelled pressure and temperature trajectories and exhumation velocities of selected crustal units agree with estimates for the Western Alps. The presented models support previous hypotheses of synconvergent exhumation, but do not rely on excessive erosion or divergent plate motion. Thus, our predictions provide new insights into processes leading to the exhumation of coherent (U)HP crustal units consistent with observations and natural data from the Western Alps.

Download Full-text

Geochemistry and geochronology of OIB-type, Early Jurassic magmatism in the Zhangguangcai range, NE China, as a result of continental back-arc extension

Geological Magazine ◽

10.1017/s0016756818000705 ◽

2018 ◽

Vol 158 (1) ◽

pp. 143-157 ◽

Cited By ~ 4

Author(s):

Guangying Feng ◽

Yildirim Dilek ◽

Xiaolu Niu ◽

Fei Liu ◽

Jingsui Yang

Keyword(s):

East Asia ◽

Trace Element ◽

Partial Melting ◽

Early Jurassic ◽

Ne China ◽

Mafic Rocks ◽

Asthenospheric Mantle ◽

Trace Element Contents ◽

Pacific Slab ◽

Back Arc

AbstractThe Zhangguangcai Range in the Xing’an Mongolian Orogenic Belt, NE China, contains Early Jurassic (c. 188 Ma) Dabaizigou (DBZG) porphyritic dolerite. Compared with other island-arc mafic rocks, the DBZG dolerite is characterized by high trace-element contents, relatively weak Nb and Ta enrichments, and no Zr, Hf or Ti depletions, similar to OIB-type rocks. Analysed rocks have (87Sr/86Sr)i ratios of 0.7033–0.7044, relatively uniform positive ɛNd(t) values of 2.3–3.2 and positive ɛHf(t) values of 8.5–17.1. Trace-element and isotopic modelling indicates that the DBZG mafic rocks were generated by partial melting of asthenospheric mantle under garnet- to spinel-facies conditions. The occurrence of OIB-like mafic intrusion suggests significant upwelling of the asthenosphere in response to lithospheric attenuation caused by continental rifting. These processes occurred in an incipient continental back-arc environment in the upper plate of a palaeo-Pacific slab subducting W–NW beneath East Asia.

Download Full-text