Structure and composition of the subcontinental lithospheric mantle in convergent settingsinsights from mantle xenoliths hosted in alkaline magmatism

Abstract

The mains goal of this Thesis has been to investigate the structure and geochemical composition and evolution of the subcontinental lithospheric mantle (SCLM) beneath convergent settings and that are recorded in mantle xenoliths hosted in postorogenic alkaline magmatism. The Thesis investigates two cases of study: (A) SCLM involving circumcratonic in collisional orogens around cratons (Sangilen Plateau, Tuva, Russia); and (B) SCLM recording supra-subduction and continental delamination tectonomagmatic processes related to the Tertiary Alpine evolution of the Betic-Rif Cordillera (SE Iberian Volcanic Province, Murcia, SE Spain). (A) SCLM INVOLVING CIRCUMCRATONIC MANTLE IN COLLISIONAL OROGENS We have studied spinel peridotite xenoliths hosted in Early Ordovician alkaline lamprophyres from the Sangilen Plateau (Tuva, Russia) in the accretionary Central Asian orogenic belt (southern Siberia). These lamprophyre-hosted spinel peridotite xenoliths allowed us to investigate the composition and thermal structure of the circumcratonic subcontinental lithospheric mantle of the Tuva-Mongolian micro-continent. Sangilen mantle xenoliths therefore offer complementary information on the Central Asia subcontinental mantle lithosphere to that provided by mantle xenoliths in younger Neogene basalts that sample the subcontinental mantle. On the basis of our petrological and geochemical study of Sangilen mantle xenoliths, the following conclusions could be drawn: (i.) Sangilen xenoliths record a narrow range of equilibration temperature (ca. 1000¿1100 °C) plotting along a relatively hot intracontinental geotherm at lithospheric depths from ~43 to 53 km (1.3 to 1.6 GPa); (ii.) Their variation of olivine Mg# (87.9¿90.9) with spinel Cr# (9.5¿45.7) indicates xenoliths are mostly residues of up to 10% melting of a depleted peridotite source; (iii.) The elevated YbN content of clinopyroxene (sample 5H-10) and the variable fractionation of LREE-MREE relative to HREE in clinopyroxene from other xenoliths indicate they underwent variable metasomatic processes. These processes probably consisted of episodes of percolation-reaction through peridotite of small-melt fractions of alkaline mafic melts precursor to Agardag alkaline lamprophyres; (iv.) The lack of correlation with depth of modal variations, textural types, and the inferred degrees of melting and trace element patterns of clinopyroxene, indicates the absence of a texturally or compositionally layering in the lithospheric mantle at the time of his sampling by Ordovician lamprophyres. As shown by numerical percolation-reaction models, the observed compositionally variation of xenoliths is better accounted by depleted lithosphere variably metasomatized along a network of percolating alkaline mafic melts heterogeneously distributed throughout the Sangilen lithospheric mantle section. (B) SCLM IN SUPRA-SUBDUCTION AND CONTINENTAL DELAMINATION SETTINGS Plio-Pleistocene Alkali basalts¿2-3 Ma¿in the inner easternmost Betic Cordillera, have entrained numerous mantle xenoliths that provide a snapshot of the structure and composition of the lithospheric mantle beneath the NE termination of the Alboran arc system in the western Mediterranean. They also record the deformation of the lithospheric mantle and yield complementary information to seismological observations. A synthesis of available in situ and whole-rock Os-isotope data on mantle-derived peridotites shows that maximum TRD ages of ca. 1.8 Ga is widespread in the subcontinental mantle of Europe and Africa outboard from the Betics-Maghrebides-Apennines front. In contrast, the mantle enclosed within the Alpine domain records TRD as old as 2.6 Ga in Tallante, revealing a previously unrecognized Archean domain or domains in the Central/Western Mediterranean. Our observations indicate that ancient fragments of subcontinental lithospheric mantle have played an important role in the development of the present architecture of the Mediterranean lithosphere. Re-depletion model ages of sulfides in peridotite xenoliths and U-Pb ages in lower crust xenoliths and anatectic melts in the Eastern Betics yield inherited Neoarchean ages similar to those reported in the Ronda orogenic peridotite and lower crust in the western Betics Alpujárrides. These ages indicates a non-Iberian provenance of the lower crust and subcrustal lithosphere in the eastern Betic. Despite their common Alboran provenance, eastern Betics mantle xenoliths sampled a more fertile mantle lithosphere recording a younger and distinct deformation regime than that preserved in the Ronda peridotite. The detailed microstructural and crystal preferred orientation study of xenoliths provide important insights into the origin and deformation record of the mantle in the westernmost Mediterranean. CPOs of orthopyroxene in orthopyroxenite veins of composite xenoliths¿formed at 2.2-4.4 Ma by silica-rich fluid/melt reaction with mantle peridotite¿indicates that metasomatism was synkinematic with high-T ductile dislocation creep flow of mantle peridotite, demonstrating that ductile fabrics were developed sometime in the late Neogene. Peridotite xenoliths show a distinct olivine axial-[100] pattern that is characteristic of deformation under simple shear in a transtensional tectonic regime. Texture and composition of xenoliths record decompression from depths of at least 55-60 km up to shallow subcrustal depths of 19-20 km. Synkinematic overprinting of mineral assemblages from the garnet-spinel to the plagioclase facies demonstrates that this 36-40 km uplift was continuously accommodated by ductile shear thinning of the lithospheric mantle. Lithospheric thinning had minimum stretching beta factors of ca. 1.5 and long-term high strain rates of ca. 9-7¿10-13 s-1, values that are characteristics of rapidly extending continental lithosphere with geothermal gradient in excess of 100 mW¿m-2. As constrained by the calculate seismic properties of Eastern Betic mantle xenoliths, the N 69º strike of SKS and Pn seismic anisotropy in the eastern Betics can be explained by a shallow lithospheric mantle fabric with olivine [100]-axes subhorizontal and subparallel to the mantle flow direction. The current shallow mantle lithosphere thickness (40-80 km) and measured delays of SKS waves in the eastern Betics can only be accounted for by a steeply dipping mantle foliation and subhorizontal lineation; this geometry of the lithospheric mantle peridotite fabric implies active or frozen mantle flow with a dominantly strike-slip component subparallel to the paleo-Iberian margin. This configuration may reflect strike-slip deformation in the early Miocene during W-SW slab retreat of the Alboran lithosphere at the edge of the Iberian margin. Compositional profiles in xenoliths indicate that initiation of ductile thinning of the subcontinental mantle may have occurred sometime between 6.2 and 9.4 Ma. This time span broadly overlaps with deep processes events in the eastern Betics such as onset of lower crust anatexis (ca. 9 Ma) and the change from subduction-related to intraplate-type volcanism (6.3-4.8 Ma). These magmatic events are synchronous with uplift associated with the Messinian salinity crisis that some authors ascribe to asthenospheric upwelling and lithospheric edge delamination of the Iberian margin during westward rollback of subducted Tethys oceanic lithosphere. Edge delamination of the Iberian margin mantle thinning and decompression of hot extended lithospheric of Alboran provenance and its lateral westward inflow under the detached margins to fill the detached cold lithosphere following the inherited S-SW lithospheric anisotropy. This process would explain flat Moho, thin lithosphere, heating, crustal anatexis, shift of magmatism, and uplift in the late Miocene and Pliocene in the Eastern Betics. Detachment of the subducted lithospheric mantle and westward replacement by Alboran-type hot lower lithosphere would account for Alboran provenance of mantle and lower crustal in the Eastern Betics. This geological paroxysm was coeval to middle to late Miocene E-W opening and oceanic magmatic accretion of the Algero-Balearic basin. The existence of strike-slip inherited mantle fabrics may continue governing some deep processes in the Eastern Betics. Detailed geochemical study of whole rock and mineral major- and trace elements, as well as Nd-Sr-Pb isotopic composition data on xenoliths provide key information on how these Alpine tectono-magmatic processes have affected the westernmost Mediterranean subcontinental mantle. The whole rock major element compositional variation, along with by their mineral chemistry, reflects increasing fertility from clinopyroxene-poor peridotites (Group I; Mg# up to 0.915), common lherzolites (Group II; Mg# up to 0.906) and fertile lherzolites (Group III; Mg# of 0.868-0.889) to wehrlites (Mg# of 0.867-0.874). The mineral major element chemistry records the geochemical imprint of maximum 10-12 % partial melting in the most depleted Group I peridotites. Trace element and isotopic data attest for various degrees of metasomatic enrichment that overprinted the previously depleted lithospheric mantle. Interaction with melts produced enrichment of LREE in Group II and Group III peridotites, as well as wehrlites. In contrast to major and trace elements, Nd-Sr-Pb radiogenic isotope systematic is unrelated to compositional groups and shows isotopic variations between DMM to EM2 end-member and contribution of an Atlantic sediment-like component. These geochemical evidences attest for the percolation of slab-derived, SiO2-undersaturated melts (and hydrous fluids) with a sediment affinity in the supra-subduction continental lithospheric mantle beneath the Alboran Basin, possibly released from the subducted Tethys oceanic lithosphere.