Mineralogy of a High-Temperature Skarn, in High CO2 Activity Conditions: The Occurrence from Măgureaua Vaţei (Metaliferi Massif, Apuseni Mountains, Romania)

A shallow-level monzodioritic to quartz-monzodioritic pluton of the Upper Cretaceous age caused contact metamorphism of Tithonic–Kimmeridgian reef limestones at Măgureaua Vaţei (Metaliferi Massif, Apuseni Mountains, Romania). The preserved peak metamorphic assemblage includes gehlenite (Ak 33.64–38.13), monticellite, wollastonite-2M, Ti-poor calcic garnet, and Ca-Tschermak diopside (with up to 11.15 mol.% Ca-Tschermak molecule). From the monzodioritic body to the calcitic marble, the periplutonic zoning can be described as: monzodiorite/agpaitic syenite-like inner endoskarn/wollastonite + perovskite + Ti-poor grossular + Al-rich diopside/wollastonite + Ti-poor grossular + diopside + vesuvianite/gehlenite + wollastonite + Ti poor grossular + Ti-rich grossular (outer endoskarn)/wollastonite + vesuvianite + garnet (inner exoskarn)/wollastonite + Ti-rich garnet + vesuvianite + diopside (outer exoskarn)/calcitic marble. Three generations of Ca garnets could be identified, as follows: (1) Ti-poor grossular (Grs 53.51–81.03 mol.%) in equilibrium with gehlenite; (2) Ti-rich grossular (Grs 51.13–53.47 mol.%, with up to 19.97 mol.% morimotoite in solid solution); and (3) titanian andradite (Grs 32.70–45.85 mol.%), with up to 29.15 mol.% morimotoite in solid solution. An early hydrothermal stage produced retrogression of the peak paragenesis toward vesuvianite, hydroxylellestadite (or Si-substituted apatite), clinochlore, “hibschite” (H4O4-substituted grossular). A late hydrothermal event induced the formation of lizardite, chrysotile, dickite, thaumasite, okenite and tobermorite. A weathering paragenesis includes allophane, C-S-H gels and probably portlandite, unpreserved because of its transformation in aragonite then calcite. Overprints of these late events on the primary zoning are quite limited.

Alpine oxidation of lithium micas in Permian S-type granites (Gemeric unit, Western Carpathians, Slovakia)

Lithium micas of the zinnwaldite and phengite–Li-phengite series occur as characteristic minerals in Permian Li-F-(P) granites of the western Gemeric unit (Western Carpathians) accompanied by topaz, tourmaline, Nb, Ta, Ti, Sn oxides and aluminophosphates. The calculated Li2O contents of all the mica analysed, together with Rb2O and Cs2O were confirmed by LA-ICP-MS analyses for all the identified micas. Samples from three localities were investigated: two surficial (Surovec, Vrchsúl’ová); and one drill hole (Dlhá dolina). Zinnwaldite (polylithionite) occurs in the upper level of the Dlhá dolina granitic intrusion and in the nearby shallow satellite body of Surovec. The lower level porphyritic granites contain only siderophyllite. The Vrchsúl’ ová micas are closer in composition to Li-annite and siderophyllite. Dioctahedral micas are mostly phengites, although zinnwaldite-bearing granites are rich in late-crystallizing Li-phengite, which extensively replaces earlier zinnwaldite. The secondary Liphengite and phengite are interpreted as products of Alpine metamorphism during Cretaceous burial and subsequent exhumation of the Gemeric unit. Reactions are suggested explaining the formation of Li-phengite by reaction of zinnwaldite with phengite or with muscovite. All mica types were investigated by Mössbauer spectroscopy, which showed high degrees of oxidation (25–50% Fe3+ of total Fe) with the exception of zinnwaldite from Vrchsúl’ová, which may have preserved an original, reduced value of 10%. The metamorphic assemblage present permitted calculation of P-T-X conditions: T = 184°C, P = 320 MPa, with oxidation of siderophyllite to phengite + goethite and fO2 at ΔNN = 4.7, confirming the low-grade conditions of the Alpine metamorphism in agreement with previous estimates.

Metamorphic record in metalherzolite pockets within the Virsini metaharzburgite from the Kechros HP metamorphic complex in Eastern Rhodope, Greece.

The Virsini antigorite serpentinite from the Kechros HP metamorphic complex in eastern Rhodope is a serpentinized harzburgite with pockets of lherzolitic composition. In the metalherzolites the mantle assemblage is Ol-1+Opx+Cpx-1+Cr-Spl. The metamorphic assemblage is Atg+Ol-2+Cpx-2+Ol-3+Chl+Tr+Cr-Mag. Antigorite and Cr-magnetite formed in an early stage of the metamorphic event. With increase in metamorphic grade olivine (Ol-2) was formed at the expense of antigorite and with further increase tremolite, olivine (Ol-3) and chlorite were formed at the expense of clinopyroxene and antigorite. P-T conditions of ≈1.7 GPa/≈570 oC are recorded in neighbouring eclogites. For the temperature of 570 oC the reaction Di+Atg→Ol+Tr+H2O occurs at pressure of ~1.2 GPa suggesting that the stability field of the mineral assemblage Atg+Ol+Tr in the metalherzolite is crossed during decompression.

Kinematically unrelated C—S fabrics: an Lexample of extensional shear band cleavage from the Veporic Unit (Western Carpathians)

Discontinuous and kinematically unrelated C-S fabrics have been recognized along the contact between the Gemeric and Veporic Units in the Western Carpathians. The formation of S and C fabrics within orthogneiss, quartzite and chloritoid-kyanite schist of the Veporic Unit is associated with Cretaceous syn-burial orogen-parallel flow and subsequent exhumational unroofing. The formation of the two fabrics characterized by distinct quartz deformation microstructure and metamorphic assemblage is separated by an inter-tectonic growth of transversal chloritoid-, kyanite-, ± monazite-bearing assemblage. The monazite U-Th-Pb concordia age of 97 ± 4 Ma was obtained by the laser ablation ICP-MS dating method. The age of this inter-tectonic metamorphic stage together with existing 40Ar/39Ar ages on exhumation of the Veporic Unit indicate that despite the similar appearance to shear bands or C-S mylonites there is a time span of at least 10 Myr between the formation of homogeneous S fabrics and superposed discrete C fabrics in the studied rocks

Mineral chemistry and thermobarometry of the staurolite-chloritoid schists from Poshtuk, NW Iran

The Poshtuk metapelitic rocks in northwestern Iran underwent two main phases of regional and contact metamorphism. Microstructures, textural features and field relations indicate that these rocks underwent a polymetamorphic history. The dominant metamorphic assemblage of the metapelites is garnet, staurolite, chloritoid, chlorite, muscovite and quartz, which grew mainly syntectonically during the later contact metamorphic event. Peak metamorphic conditions of this event took place at 580 °C and ~3–4 kbar, indicating that this event occurred under high-temperature and low-pressure conditions (HT/LP metamorphism), which reflects the high heat flow in this part of the crust. This event is mainly controlled by advective heat input through magmatic intrusions into all levels of the crust. These extensive Eocene metamorphic and magmatic activities can be associated with the early Alpine Orogeny, which resulted in this area from the convergence between the Arabian and Eurasian plates, and the Cenozoic closure of the Tethys oceanic tract(s).

Thermochronology of the Needle Falls Shear Zone: a post-collisional high-strain zone of the Trans-Hudson Orogen

The Needle Falls Shear Zone is a major Paleoproterozoic discontinuity separating an Archean craton from accreted Paleoproterozoic terranes in the western Trans-Hudson Orogen. It is a 1 km-wide subvertical mylonite zone forming the southwestern sector of a northeast-trending shear system extending for more than 400 km. Kinematic data indicate oblique, east-side-up dextral strike-slip displacement throughout its evolution from ductile through late brittle deformation. For most of its exposed length, it marks the northwestern margin of the Wathaman Batholith. The oldest Pb-evaporation ages of zircons from mylonite average 1857 Ma, consistent with published zircon ages of the Wathaman Batholith, confirming the batholith is protolith for much of the shear zone. The synkinematic metamorphic assemblage of amphibole–biotite–quartz–feldspar signifies deformation under amphibolite-facies conditions (500–630°C). Oxygen isotope fractionations between quartz–hornblende and quartz–biotite pairs in textural equilibrium give temperatures from 430°C to 610°C, with a mode at 550°C. For hornblende–biotite pairs, 40Ar/39Ar plateau ages of hornblende cluster at 1780 Ma and of biotite at 1757 Ma. Given textural and oxygen isotope equilibrium between hornblende, biotite, and quartz, these ages are interpreted as cooling ages; consequently, this sector of the Trans-Hudson Orogen cooled from 500°C to 300–350°C over about 23 million years following ductile deformation. The timing of ductile deformation cannot be well constrained, but occurred after emplacement of the Wathaman Batholith at 1857 Ma and prior to cooling of amphibole below its Ar blocking temperature at 1780 Ma.

The emplacement of peridotites and associated oceanic rocks from the Lizard Complex, southwest England

Upper mantle peridotites and associated oceanic rocks from the Lizard Complex, southwest England, preserve evidence for a multistage geological history. Steeply dipping pre-emplacement fabrics record high-temperature (900–1100°C) shearing and exhumation of the mantle peridotites apparently formed during localized NE–SW rifting in a pull-apart basin setting (c. 400–390 Ma). Associated oceanic rocks (Landewednack amphibolites) preserve a pre-emplacement prograde brown amphibole-bearing metamorphic assemblage and steeply dipping fabric thought to have formed as the newly formed oceanic crust was juxtaposed with newly exhumed hot mantle peridotite during NE–SW rifting. In both the peridotites and Landewednack amphibolites, steep pre-emplacement structures are cross-cut by low-angle mylonitic fabrics thought to have formed during the initial phases of emplacement of mantle over crustal rocks in a partially intra-oceanic setting (c. 390–375 Ma). The fabrics in peridotites and amphibolites exhibit retrograde mineral assemblages (c. 500–800°C), with the amphibolites preserving two superimposed assemblages, green amphibole + titanite and colourless magnesio-hornblende, respectively, that are thought to record progressive down-temperature deformation during thrusting. Emplacement-related structures in both the basal peridotites and amphibolites consistently dip at low to moderate angles NW, with down-dip lineations and kinematic indicators showing consistent top-to-the-NW senses of shear. Syn-emplacement magmatism is recorded by intrusions of foliated Kennack Gneiss. Anastomosing serpentine-filled faults mark many existing low-angle contacts between the peridotites and Landewednack amphibolites and appear to represent the final, lowest-temperature (< 250°C) stages of emplacement (c. 370 Ma). This study shows that ‘dynamothermal aureoles’ in ophiolites may preserve evidence for tectonothermal events that pre-date thrust emplacement.