Petrography and Geochemistry of the Leucocratic Rocks in the Ophiolites from the Pollino Massif (Southern Italy)

In the Tethyan realm, leucocratic rocks were recognized as dikes and layers outcropping in the ophiolitic rocks of the Western Alps, in Corsica, and in the Northern Apennines. Several authors have suggested that the origin of leucocratic rocks is associated with partial melting of cumulate gabbro. Major and trace elements composition and paragenesis provided information about the leucocratic rocks genetic processes. This research aims at disclosing, for the first time, the petrographical and geochemical features of Timpa delle Murge leucocratic rocks, Pollino Massif (southern Italy), in order to discuss their origin and geodynamic significance through a comparison with other Tethyan leucocratic rocks. These rocks are characterized by high amounts of silica with moderate alumina and iron-magnesium contents showing higher potassium contents than plagiogranites, due to plagioclase alteration to sericite. Plagioclase fractionation reflects negative Eu anomalies indicating its derivation from gabbroic crystal mushes. The chondrite normalized REEs patterns suggest the participation of partial melts derived from a metasomatized mantle in a subduction environment. The results reveal some similarities in composition with other Tethyan leucocratic rocks, especially those concerning Corsica and the Northern Alps. These new data provide further clues on the origin of these leucocratic rocks and the Tethyan area geodynamic evolution.

Temporal and Petrogenetic Links Between Mesoproterozoic Alkaline and Carbonatite Magmas at Mountain Pass, California

Mountain Pass is the site of the most economically important rare earth element (REE) deposits in the United States. Mesoproterozoic alkaline intrusions are spatiotemporally associated with a composite carbonatite stock that hosts REE ore. Understanding the genesis of the alkaline and carbonatite magmas is an essential scientific goal for a society in which critical minerals are in high demand and will continue to be so for the foreseeable future. We present an ion microprobe study of zircon crystals in shonkinite and syenite intrusions to establish geochronological and geochemical constraints on the igneous underpinnings of the Mountain Pass REE deposit. Silicate whole-rock compositions occupy a broad spectrum (50–72 wt % SiO2), are ultrapotassic (6–9 wt % K2O; K2O/Na2O = 2–9), and have highly elevated concentrations of REEs (La 500–1,100× chondritic). Zircon concordia 206Pb/238U-207Pb/235U ages determined for shonkinite and syenite units are 1409 ± 8, 1409 ± 12, 1410 ± 8, and 1415 ± 6 Ma (2σ). Most shonkinite dikes are dominated by inherited Paleoproterozoic xenocrysts, but there are sparse primary zircons with 207Pb/206Pb ages of 1390–1380 ± 15 Ma for the youngest grains. Our new zircon U-Pb ages for shonkinite and syenite units overlap published monazite Th-Pb ages for the carbonatite orebody and a smaller carbonatite dike. Inherited zircons in shonkinite and syenite units are ubiquitous and have a multimodal distribution of 207Pb/206Pb ages that cluster in the range of 1785–1600 ± 10–30 Ma. Primary zircons have generally lower Hf (<11,000 ppm) and higher Eu/Eu* (>0.6), Th (>300 ppm), Th/U (>1), and Ti-in-zircon temperatures (>800°C) than inherited zircons. Oxygen isotope data reveals a large range in δ18O values for primary zircons, from mantle (5–5.5‰) to crustal and supracrustal (7–9‰). A couple of low-δ18O outliers (2‰) point to a component of shallow crust altered by meteoric water. The δ18O range of inherited zircons (5–10‰) overlaps that of the primary zircons. Our study supports a model in which alkaline and carbonatite magmatism occurred over tens of millions of years, repeatedly tapping a metasomatized mantle source, which endowed magmas with elevated REEs and other diagnostic components (e.g., F, Ba). Though this metasomatized mantle region existed for the duration of Mountain Pass magmatism, it probably did not predate magmatism by substantial geologic time (>100 m.y.), based on the similarity of 1500 Ma zircons with the dominantly 1800–1600 Ma inherited zircons, as opposed to the 1450–1350 Ma primary zircons. Mountain Pass magmas had diverse crustal inputs from assimilation of Paleoproterozoic and Mesoproterozoic igneous, metaigneous, and metasedimentary rocks. Crustal assimilation is only apparent from high spatial resolution zircon analyses and underscores the need for mineral-scale approaches in understanding the genesis of the Mountain Pass system.

Short-lived alkaline magmatism related to Réunion plume in the Deccan large igneous province: inferences from petrology, 40Ar/39Ar geochronology and paleomagnetism of lamprophyre from the Sarnu-Dandali alkaline igneous complex

Existing geochronological information on Deccan indicates prolonged (started at 68.5 Ma) alkaline magmatism related to the Réunion mantle plume based on the 40Ar/39Ar ages from Sarnu-Dandali and Mundwara alkaline complexes. We studied in detail an alkaline lamprophyre, from the Sarnu-Dandali complex, rich in groundmass (magmatic) as well as xenocrystic phlogopites and clinopyroxenes. 40Ar/39Ar age determinations of the phlogopites from this lamprophyre, reveal two distinct ages of 65.44±1.5 Ma and 68.17±1Ma. However, paleomagnetic results show a VGP at 32.31 N and 298.52 E concordant with that of the Deccan Super Pole at 65.5 Ma and support the younger eruption age at ca. 65.44±1.5Ma. Analyzed phlogopites lack any signs of retention of excess radiogenic Ar and yield similar inverse isochron ages, which suggests that the older age of ca. 68.17±1Ma belongs to the crystallization of xenocrystic phlogopite during mantle metasomatism. Trace element compositions support derivation of lamprophyre magma from an OIB- type enriched (metasomatized) mantle source with an involvement of phlogopite.This finding suggests that the pre-Deccan ages of ca. 68-69 Ma reported previously, may reflect the timing of metasomatism of the subcratonic lithospheric mantle during the separation of Greater-Seychelles from India at ca. ∼68.5 Ma. The absence of pre-Deccan alkaline rocks therefore indicates the short-duration (occurred between 67-65 Ma) of alkaline as well as small-volume, volatile-rich magmatism directly related to the Réunion (Deccan) plume.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5490881

Origin and significance of the antimony mineralisation associated to mafic intrusions in the Iberian Zone and the Central Armorican Domain

<p>In the Central Iberian Zone (CIZ) and its French counterpart, the Central Armorican Domain (CAD), widespread swarms of mafic dykes with various ages and compositions are known. Indeed, numerous mafic events are recognized in the late Neoproterozoic, in the Cambrian to the Ordovician, in the Ordovician to the Devonian, at the Devonian-Carboniferous boundary, in the Permian and in the Jurassic. Such a succession of mantle partial melting events, localised or generalized, may have strong consequences (i) on the composition and the homogeneity of the mantle below both the CIZ and CAD, and (ii) on the transfert of metals in the overlying crust. Moreover, the mantle below these domains must have been modified also by the subduction of large to small oceanic crusts from the Iapetus, the Rheic, the Galicia-Moldanubian and the Paleo-tethys. Although the occurrences of paleo-subductions below the CIZ and CAD remain discussed, the southern border of the CIZ, the Ossa-Morena Zone (OMZ), is considered as a suture zone resulting from a subduction followed by a collision between 390 and 360 Ma (D1), according to the 2 opposite structural vergences at the CIZ/OMZ boundary, as well as the location of a NE-dipping slab imaged by seismic profiles. In the Armorican massif, the end of subduction is also dated at 360 Ma and associated to a north-directed subduction. The trace of this subduction below the CAD is visible in the tomographic dataset. Interestingly, these two domains (CIZ and CAD) contain the largest number of Palaeozoic antimony deposits, antimony being a volatile element. In these domains, the large clustering of antimony deposits and occurrences is observed within a ca 100km wide bands along their southern parts. In the two domains, the antimony deposits are frequently spatially associated with diabase dykes. Diabase dykes and associated antimony mineralisation have been dated at 360 Ma in the CAD but remain temporally unconstrained in the CIZ. Nevertheless, since these dykes are strongly affected by the Variscan deformation a minimum age of 350 Ma is inferred. Both, the peculiar composition of these diabase dykes, relatively enriched in Cs, Li, Pb and relatively depleted in K and Rb, the spatial association with antimony at the end of a 360Ma subduction, suggest a link between antimony and a ca 360Ma mafic magmatism which could result from the partial melting of a subduction-related metasomatized mantle.</p><p>This work was funded by the ANR (ANR-19-MIN2-0002), the AEI (MICIU/AEI/REF.: PCI2019-103779), the FCT (ERA-MIN/0005/2018) and author’s institutions in the framework of the ERA-MIN2 AUREOLE project (https://aureole.brgm.fr).</p>

Supplemental Material: Progressive spatial and temporal evolution of tectonic triggers and metasomatized mantle lithosphere sources for orogenic gold mineralization in a Triassic convergent margin: Kunlun-Qinling Orogen, central China

Figures of deposit geology and ore assemblages and the compiled ages and isotopes in the East Kunlun-West Qinling Orogen, China.

Supplemental Material: Progressive spatial and temporal evolution of tectonic triggers and metasomatized mantle lithosphere sources for orogenic gold mineralization in a Triassic convergent margin: Kunlun-Qinling Orogen, central China

Figures of deposit geology and ore assemblages and the compiled ages and isotopes in the East Kunlun-West Qinling Orogen, China.

Chapter 1.3 Antarctic volcanism: petrology and tectonomagmatic overview

Petrological investigations over the past 30 years have significantly advanced our knowledge of the origin and evolution of magmas emplaced within and erupted on top of the Antarctic Plate. Over the last 200 myr Antarctica has experienced: (1) several episodes of rifting, leading to the fragmentation of Gondwana and the formation by c. 83 Ma of the current Antarctica Plate; (2) long-lived subduction that shut down progressively eastwards along the Gondwana margin in the Late Cretaceous and is still active at the northernmost tip of the Antarctic Peninsula; and (3) broad extension across West Antarctica that produced one of the Earth's major continental rift systems. The dynamic tectonic history of Antarctica since the Triassic has led to a diversity of volcano types and igneous rock compositions with correspondingly diverse origins. Many intriguing questions remain about the petrology of mantle sources and the mechanisms for melting during each tectonomagmatic phase. For intraplate magmatism, the upwelling of deep mantle plumes is often evoked. Alternatively, subduction-related metasomatized mantle sources and melting by more passive means (e.g. edge-driven flow, translithospheric faulting, slab windows) are proposed. A brief review of these often competing models is provided in this chapter along with recommendations for ongoing petrological research in Antarctica.