Claudio Ghezzo

Minette inclusions in the rhyodacitic lavas of Mt-Amiata (Central Italy): mineralogical and chemical evidence of mixing between Tuscan and Roman type magmas

Abstract Major-element, trace-element and mineralogical data are reported for a series of mafic inclusions occurring in the rhyodacitic lavas of Mt. Amiata (southern Tuscany, Italy), a volcanic complex belonging to the Tuscan Magmatic Province. The inclusions form, together with some subordinate mafic latitic lavas and the host rock rhyodacites, a continuous series, which varies from olivine and minor nepheline normative to quartz, hypersthene and corundum normative rock-types over a range of 48–67% SiO 2 . All of the rocks are characterized by high contents of potassium (5–6% K 2 O) and many trace elements, particularly the incompatible elements. Following straight-line correlation trends, Al, Fe, Mg, Ca, Ti, P, Sr and Ba decrease with increasing SiO 2 , whereas Na and Li increase, and K, Rb, Zr, La, Ce, Nb and Y remain virtually constant. The primary mineral assemblage of the inclusions consists of diopsidic pyroxene, forsteritic olivine and mica, and is representative for minettes. Xenocrysts of plagioclase, sanidine, orthopyroxene clinopyroxene, biotite, ilmenite and olivine occur in increasing amounts in the more siliceous samples. The composition of these xenocrysts is identical to that of the phenocryst assemblage in the rhyodacites. The latitic lavas are rich in xenocrysts and differ from the minettes mainly by the absence of primary mica. The data strongly suggest a magma mixing origin for the minettes, latites and possibly part of the rhyodacites. Injection of mafic magma into a siliceous magma chamber has resulted in the formation of hybrid inclusions and a latitic magma that could only reach the surface after eruption of the voluminous rhyodacitic lavas. The rhyodacites which erupted early and are slightly richer in SiO 2 than most of the other rhyodacites are considered to represent the acid mixing component, whereas the mafic end-member has close affinities to the potassic alkaline lavas of the adjacent Roman Province. The inclusions have the modal composition of a minette and this implies that volatiles were important constituents of the hybrid magmas. This may be attributed to the primary character of the Roman-type end-member involved, or explained by a process of selective enrichment operating during mixing. Although upper mantle derived Roman magmas have interacted with Tuscan magmas of crustal anatectic origin at Mt. Amiata and probably other volcanic centres in central Italy, the specific chemistry of lavas in both provinces cannot be explained by simple mixing of these magmas in shallow chambers.

Geochemistry of granitic rocks from the Hercynian Sardinia-Corsica batholith: Implication for magma genesis

Abstract The Sardinia-Corsica batholith was structured in the late stage of the Hercynian orogenesis. The granitoids intrude mainly metamorphic complexes grading from zeolite up to amphibolite facies. The batholith is heterogeneous consisting of complexes with different affinity, chemical composition, age and degree of deformation. The present paper reports major-and trace-element data for selected samples coming only from Sardinian outcrops. The rocks range from gabbro-diorite to tonalite, monzogranite and leucogranite. The two latter lithologies are the most abundant, gabbrodiorites and tonalites occurring in minor amounts and mainly in northern-central Sardinia. Over 75% of the granitoids contain microgranular enclaves of magmatic origin. The age of the rocks falls in the interval between 307 and 281 Ma. Sr isotope initial ratios are high, ranging between 0.7083 and 0.7107. REE, Rb, Sr, Ba, Zr, Th, Ta, Hf, Co and Sc abundances were determined on selected samples. All elements follow three types of trends vs. CaO, which is used as differentiation index. Two trends show positive and negative correlations while the third one shows a bell-shaped pattern. LREE have different degrees of enrichment (La = 20−120× ch) and HREE show variable fractionation with prevailing ( Tb Yb ) n . The two peraluminous samples have very different geochemical characteristics. From the geochemical point of view all the rocks coming from the Sardinian segment of the batholith display a typical calc-alkaline chemical character showing the imprint of both “normal and mature” continental arc geodynamic environments. Geochemical trends suggest some petrogenetic constraints. The complete sequence of differentiation can be neither the product of crystal/liquid fractionation processes starting from a single basic parent magma nor the product of an AFC process. On the contrary, a two-stage model can be proposed. In the first stage a mafic melt of subcrustal origin interacted with monzogranitic magmas derived from 25–35% degree of melting of a crustal biotite amphibolitic source. Such a mixing process acted together with a crystal/liquid fractionation process to give tonalites and granodiorites. In the second stage lesser degrees of melting of the same crustal source could give the late-stage leucogranitic masses. A possible scenario, able to take into account field and geochemical data, can be suggested for the genesis of this suite and we propose it as a working model for future investigations.

Crystal chemistry of Al-rich biotites coexisting with muscovites in peraluminous granites

Abstract A comparison was made between single-crystal structure refinements, electron microprobe analyses and octahedral site populations of seven biotite crystals and data obtained previously for coexisting muscovite in peraluminous granites using the same methods. Both micas, from several plutons of Northern Victoria Land (Antarctica) and Sardinia (Italy), show significant octahedral substitutions: biotite has a composition characterized by relatively high [6]Al content (0.32 ≤ [6]Al ≤ 0.59 apfu), whereas muscovite is characterized by phengite-like substitutions [0.12 ≤ [6](Mg + Fe + Ti + Mn) ≤ 0.35]. Mean bond-lengths and electron count data for six biotites-1M (space group C2/m, agreement factor 2.7% ≤ Robs ≤ 3.6%) and a biotite-2M1 (space group C2/c, Robs = 2.8%) show that Al substitutes for divalent cations in the octahedral M2 site and that the Fe and Mg distribution is disordered. The mean tetrahedral bond lengths determined for biotite-2M1 reflect Al-Si disorder. In coexisting muscovite-2M1 crystals, small positive electron density residuals close to M1 site position as well as the increase in M2 mean atomic number is in agreement with the presence of a significant phengitic component. Reduction in biotite unit-cell dimensions with the increase of Al follows a pattern similar to that of associated muscovites, and the octahedral site volumes of both micas are influenced by the Al saturation index (ASI) of the rock. These results, and the calculated partition coefficients between biotite and muscovite for elements in M sites, are consistent with continuous reaction and re-equilibration of biotite and muscovite during crystallization of peraluminous granitic melts.

Evidence of magmatic CO2-rich fluids in peraluminous graphite-bearing leucogranites from Deep Freeze Range (northern Victoria Land, Antarctica)

Fine-grained peraluminous synkinematic leuco-monzogranites (SKG), of Cambro-Ordovician age, occur as veins and sills (up to 20–30 m thick) in the Deep Freeze Range, within the medium to high-grade metamorphics of the Wilson Terrane. Secondary fibrolite + graphite intergrowths occur in feldspars and subordinately in quartz. Four main solid and fluid inclusion populations are observed: primary mixed CO2+H2O inclusions + Al2SiO5 ± brines in garnet (type 1); early CO2-rich inclusions (± brines) in quartz (type 2); early CO2+CH4 (up to 4 mol%)±H2O inclusions + graphite + fibrolite in quartz (type 3); late CH4+CO2+N2 inclusions and H2O inclusions in quartz (type 4). Densities of type 1 inclusions are consistent with the crystallization conditions of SKG (≈750°C and 3 kbar). The other types are post-magmatic: densities of type 2 and 3 inclusions suggest isobaric cooling at high temperature (≈700–550°C). Type 4 inclusions were trapped below 500°C. The SKG crystallized from a magma that was at some stage vapour-saturated; fluids were CO2-rich, possibly with immiscible brines. CO2-rich fluids (±brines) characterize the transition from magmatic to post-magmatic stages; progressive isobaric cooling (T<670°C) led to a continuous decrease offO2 can entering in the graphite stability field; at the same time, the feldspars reacted with CO2-rich fluids to give secondary fibrolite + graphite. Decrease ofT andfO2 can explain the progressive variation in the fluid composition from CO2-rich to CH4 and water dominated in a closed system (in situ evolution). The presence of N2 the late stages indicates interaction with external metamorphic fluids.

Granite-lamprophyre connection in the latest stages of the Early Paleozoic Ross Orogeny (Victoria Land, Antarctica)

The Ross Orogen of the Transantarctic Mountains developed in response to the early Paleozoic convergence between the paleo–Pacific plate and the Antarctic margin of Gondwana. The central Victoria Land sector of the orogen is characterized by the widespread occurrence of pink Irizar granite plutons and dikes and Vegetation lamprophyric dikes and sills, which were emplaced in a tensional regime during a restricted time interval of the latest Ross Orogeny, ca. 490 Ma, as documented by new geochronological zircon U-Pb and mineral 40 Ar- 39 Ar data. The syenomonzogranitic Irizar granites-dikes and the Vegetation lamprophyres are all potassic and, despite the chemical gap between them, have overlapping 87 Sr/ 86 Sr (490 Ma) and ϵ Nd (490 Ma) values, within 0.7074–0.7092 and −4.4 to −7.5, respectively. The genesis of Vegetation lamprophyres can be ascribed to the melting of previously enriched subcontinental lithospheric mantle further metasomatized by a subduction component during the Ross convergence. Melting was probably linked to asthenospheric upwelling during postcollisional slab rollback and convective thinning and/or delamination of overthickened lithosphere. On the other hand, the overlap of age, geochemical, and Sr-Nd isotope data between Vegetation and Irizar products, supported by geochemical modeling, suggests that the Irizar felsic magmas were derived by partial remelting of underplated material similar in composition to the Vegetation lamprophyres. This scenario provides new insights into the genesis of widespread postcollisional granites in orogens worldwide, implying significant net crustal growth by magma underplating in the very latest orogenic stages. In the regional geodynamic framework, the NE strike shared by both mafic and felsic dikes along 300 km of the convergent margin points to NW-SE extension, which in turn suggests oblique convergence of paleo–Pacific and Antarctic plates during the latest orogenic stages. A comparison of latest igneous activity in the Ross Orogen with contemporary magmatism in southeastern Australia-Tasmania implies that different mechanisms triggered the magmatic activity: slab rollback in the Antarctic sector of the margin versus slab tear in the Australia-Tasmania sector.

Oxygen isotope evidence for crustal assimilation and magma mixing in the Granite Harbour Intrusives, Northern Victoria Land, Antarctica

Abstract The stable isotope composition (O,H) of whole-rock and mineral separates of Cambrian-Ordovician gabbros, diorites, granodiorites and granites forming the Mt. Abbott composite intrusions (Northern Victoria Land, Antarctica) was measured to constrain the origin and evolution of the magmas postdating the Ross Orogen. The δ 18 O values of olivine gabbros plot in the field of slightly evolved mantle-derived melts ( δ 18 O WR =6.8–7.4‰). The O-isotope character of the mantle source inferred from the δ 18 O values of cumulous olivine in gabbros (5.7–6.8‰) is enriched in 18 O compared to modern arc-related magmas. Geochemical data and concurrent high δ 18 O values, and initial strontium ( 87 Sr/ 86 Sr=0.7060) and neodymium ( 143 Nd/ 144 Nd=0.5122) isotope ratios indicate that the olivine gabbros formed by crustal contamination of a primary calc-alkaline basaltic melt. The diorites have high δ 18 O values, among the highest ever measured for dioritic rocks (8.7–10.3‰), and Sr-isotope ratios that partially overlap with the adjacent and mingled felsic lithologies (0.708–0.710). The diorites have pyroxene with high, nearly constant δ 18 O values (8.2–8.6‰) that are independent from the silica content of the rocks; thus, they did not increase in response of the chemical evolution of the rocks. The diorites originated from the same primary calc-alkaline basalt experiencing different amounts of crustal contamination, and underwent different degrees of mixing with the adjacent granites, producing granodioritic facies and quartz/feldspar xenocrystic diorites. The δ 18 O, 87 Sr/ 86 Sr and 143 Nd/ 144 Nd compositions of the granites and granodiorites overlap (10.8–12.1‰, 0.7096–0.7108, 0.5119–0.5120). They are distinct from the values of the mafic rocks and indicate that gabbros and granites were not cogenetic. The granites are a separate melt component likely derived from nonmodal partial melting of fertile meta-igneous protoliths.

The Tiger Gabbro from northern Victoria Land, Antarctica: the roots of an island arc within the early Palaeozoic margin of Gondwana

Abstract: The Tiger Gabbro layered intrusion is one of the few mafic intrusive bodies found along the ancient Antarctic Gondwana margin. Major and trace element data and Sr–Nd isotope compositions for gabbronorites indicate an island arc signature for the Tiger Gabbro parental magma. This is the first evidence for an island arc from plutonic rocks in northern Victoria Land. The interpretation of the Tiger Gabbro as the roots of an Early Cambrian island arc (535 ± 21 Ma, Sm–Nd age), integrated with geochemical and geochronological data from the literature, matches the occurrence of the Glasgow volcanic rocks in the southern Bowers terrane, which possibly represents its effusive counterpart. A scenario for the early Palaeozoic Antarctic Gondwana margin is hence proposed in which the Tiger volcanic arc developed on the Robertson Bay microplate in response to subduction of the palaeo-Pacific plate. The Tiger arc igneous activity was coeval to the Wilson continental arc (represented by the Granite Harbour intrusive rocks), with the two subduction zones merging southwards into one. The migration of the Wilson arc towards the forearc region in turn generated the Middle Cambrian Bowers arc. Supplementary material: Major element, trace element, mineral and 40Ar–39Ar data are available at http://www.geolsoc.org.uk/SUP18351.

Fossil hydrothermal systems tracking Eocene climate change in Antarctica

During the Cenozoic, alkaline magmatism related to rifting in the Ross Sea embayment affected the basement of northern Victoria Land, Antarctica. Shallow intrusions supplied the necessary heat to cause groundwater circulation through permeable rocks, producing local hydrothermal systems. In the granitoid country rocks of the two adjacent areas of Mt. Monteagle and Mt. McGee, the biotite δD values range from −69‰ to −141‰ and from −66‰ to −183‰, respectively, and K-feldspars show δ 18 O values as low as 3.7‰ and 4.4‰. These values result from isotope exchange between igneous minerals and hydrothermal fluids with a dominant meteoric water component. Geological and tectonic histories of the two areas show marked similarities; therefore, different δ 18 O and δD values of hydrothermally altered rocks may be interpreted in terms of different isotopic composition of the waters feeding the hydrothermal systems. The isotopic shift of these meteoric waters indicates that climate change occurred in the time span between the 42 Ma intrusion of Mt. Monteagle and the 38 Ma intrusion of Mt. McGee, thus suggesting a cooling episode in the Ross Sea region during late Eocene time.