Geothermobarometry of Al-silicate-bearing migmatites from the Variscan chain of NE Sardinia, Italy: a P-T pseudosection approach

The inner zone of the Sardinia Variscan segment consists of two metamorphic complexes: I) A polymetamorphic Migmatite Complex, with migmatites showing polyphase anatectic processes, in the presence of kyanite or sillimanite. The Migmatite complex preserved decametric lenses of eclogite relicts (eclogites A) affected by high T, high- to intermediate P recrystallization under granulite facies conditions The decompressional garnet + Ca-clinopyroxene + amphibole ± orthopyroxene-bearing assemblages developed in granoblastic textures generally in no stress conditions. In most cases, only symplectite textures provide evidence for the eclogitic event. II) A medium grade, mostly metapelitic complex consisting of Grt, Ky, Stau-bearing micaschists and paragneisses includes quartzites and garnet-bearing amphibolite boudins with N-MORB chemical affinity. Relicts of eclogite assemblages were locally found in the metabasite (eclogites B). In eclogites A, the geothermobarometric parameters yield temperatures in the range 690°-760°C for minimum pressure A1.3 GPa. Pyroxene compositions accord with temperatures in excess of 700°C. In eclogites B, the thermometric calibrations provide temperatures in the range 610°-700°C for pressures 1.3-1.5 GPa, based on the jadeite content. The temperatures are consistent with the biotite+muscovite+garnet+kyanite+staurolite assemblage in the host paragneisses, and with lack of anatectic processes. The age of 457±2 Ma, obtained by U/Pb dating on one sample of Type A eclogite is interpreted as a minimum estimate for the magmatism of the eclogite protolith. A second zircon population defined an age of 403±4 Ma interpreted as dating the zircon crystallization during the high-grade event. The relationships between Types A and B eclogites, and their bearing on the regional framework (Sardinia, Ligurian Alps) are discussed.

The application of melt inclusion (MI) studies to migmatitic and granulitic terranes is a recent, small-scale approach for a better understanding of melting in the continental crust. In order to show the role of anatectic MI in providing a wealth of microstructural and compositional information on high-temperature metamorphism and crustal anatexis, we review a series of studies on the crustal footwall of the Ronda peridotites (Betic Cordillera, S Spain), which consists of an inverted metamorphic sequence with granulite-facies rocks showing extensive melting on top and amphibolites-facies rocks at the bottom. We studied the microstructures and geochemistry of small (2-10 mu m) primary MI hosted in peritectic garnet of metatexites at the bottom of the migmatitic sequence and of mylonitic diatexites close to the contact with the mantle rocks. The occurrence of MI is a proof that the investigated rocks were partially melted at some time in their history, despite other microstructures indicating the former presence of melt in diatexites were erased by deformation. MI show a variable degree of crystallization ranging from totally glassy to fully crystallized (nanogranites), consisting of Qtz+Pl+Kfs+Bt+Ms aggregates (often modal Kfs > P1 in diatexites). Piston cylinder remelting experiments led to the complete rehomogenization of nanogranites in metatexites at the conditions inferred for anatexis. Compositions of investigated MI are all leucogranitic and peraluminous and differ from those of coexisting leucosomes and from melts calculated by phase equilibria modeling. Systematic compositional variations have been observed between ME in metatexites and diatexites: the former commonly show higher H2O, CaO, Na2O/K2O and lower FeO. The compositions of MI in metatexites and diatexites are interpreted to record the composition of the anatectic melts produced from a peraluminous greywacke i) on, and immediately after crossing, the fluid-saturated solidus of this metasedimentary rock, and ii) during anatexis via biotite dehydration melting at increasing temperature, respectively. While partial melting at the bottom of the migmatitic sequence likely started in the presence of an aqueous fluid phase, MI data support the fluid-absent character of the melting event in diatexites. Anatectic MI should therefore be considered as a new and important opportunity to understand the partial melting processes.

Cordierite channel volatiles as evidence for dehydration melting an example from high-temperature metapelites of the Bayerische Wald (Variscan belt, Germany)

We studied a high-pressure amphibole-bearing migmatite cropping out along the northeastern coast of Sardinia, a few kilometres northeast of Olbia, in order to improve our knowledge about its evolution using pressure–temperature (P–T) pseudosections. Thermodynamic calculations with PERPLE_X were undertaken in the system Na2O–K2O–CaO–FeO–MnO–MgO–Al2O3–TiO2–SiO2–H2O using a haplogranitic melt model. Calculations were conducted for average compositions of the protolith and the tonalitic leucosome with different contents of H2O to construct various pseudosections in the P–T range 2–20 kbar and 550–900°C. We demonstrate that the molar ratios Na/K and Si/Al of the anatectic melt are of high geothermobarometric value. Particularly the melt field in the P–T pseudosection for the protolith, contoured by Si/Al isolines, shows a significant decrease in this ratio with rising pressure. This ratio is only weakly dependent on variable H2O contents of the amphibole-bearing migmatite as demonstrated by contoured T–H2O pseudosections. The application of the Na/K and Si/Al ratios in melt yielded P–T conditions close to 13 kbar and 700°C, assigned as the conditions of partial melting. Evaluation of contoured P–T pseudosections for the leucosome composition resulted in conditions of about 10.5 kbar and 700°C for the crystallization of amphibole in the leucosome melt, and 9 kbar and 680°C for complete crystallization of this melt.

Three suites of granites were produced at ca. 620-630 Ma by partial melting of a crustal section exposed as a high-grade allocthonous terrane (the Socorro-Guaxupe Thrust Nappe, SGTN) south of the Sao Francisco Craton, SE Brazil. The striking chemical and petrographical contrasts observed reflect their genesis from different protoliths under varying P, T, a(H2O). The Sao Jose do Rio Pardo (SJRP) suite is made up of folded tabular bodies with a predominant consanguineous association of foliated mangeritic to granitic rocks which resulted mostly from in situ crystal fractionation of low mg#, Ca and Sr quartz mangeritic magmas. The very low contents of radiogenic (Rb, Cs, Th, U) trace-elements and H2O inferred for their parent melts imply a depleted granulite source which, as deduced from Zr saturation thermometry, was melted under abnormally high T (up to 1,000oC). High K/Rb and Ba/Sr ratios and positive Eu anomalies seem to result from disequilibrium melting involving K-feldspar as a reactant phase, probably as a reflection of the rather dry nature of the magmas and of fast rates of melt extraction. The Pinhal-type granites occur as countless small bodies of equigranular, in part nebulitic, pink biotite granite closely associated to migmatitic gneisses. These granites are interpreted as products of biotite dehydration-melting at ca. 850°C from tonalitic-granodiorite protoliths, as indicated by their fractionated REE patterns (suggestive of residual garnet), moderately negative Eu anomalies and higher Rb/Sr as compared to the other suites (both suggestive of a plagioclaserich, biotite-poor residuum, possibly a granulite). The garnet-biotite Nazare Paulista-type granites, also closely associated to migmatitic gneisses, correspond to products of lower-temperature (700-800oC, as inferred from zircon and monazite saturation thermometry) melting of protoliths with an important metasedimentary component. Comparatively high Ca and Sr and slightly positive to negligible Eu anomalies, besides low Rb and Ba contents point to a residuum with abundant biotite and either plagioclase-poor or bearing a Ca-rich plagioclase. The widespread anatexis in this section of Neoproterozoic crust appears to reflect the influence of a huge thermal anomaly of a magnitude unusual in collisional orogens, that was responsible for the generation, in a short time interval, of granite types normally found in different tectonic environments (e.g., the SJRP mangerite-charnockite-granite suite and the peraluminous leucogranites associated to the Nazare Paulista suite).

Petrogenesis of late Hercynian calc-alkaline dykes of mid-eastern Sardinia: petrographical and geochemical data constraining hybridization process

مجموعه گرانیتوئیدی کلاه قاضی در جنوب اصفهان و در پهنه ساختاری سنندج‌- سیرجان واقع شده است. بر اساس شواهد زمین‌شناسی سن احتمالی این مجموعه ژوراسیک بالایی بوده و شامل گروه های سنگی گرانودیوریت، گرانیت و آلکالی فلدسپار گرانیت است. کانی های اصلی تشکیل‌دهنده این مجموعه شامل کوارتز، پلاژیوکلاز و فلدسپار آلکالن بوده و بیوتیت تنها کانی فرومنیزین این مجموعه نفوذی است. حضور کانی های آندالوزیت، سیلیمانیت و گارنت در این مجموعه گرانیتوئیدی نشان‌دهنده منشأ رسوبی توده های مورد بررسی است. بر اساس مطالعات صحرایی، پتروگرافی و ژئوشیمیایی، سنگ‌های مجموعه نفوذی مورد بررسی در گروه گرانیتوئیدهای نوع S قرار می گیرند و دارای ماهیت کالک‌آلکالن پتاسیم بالا و از نوع پرآلومین هستند. از لحاظ ژئوشیمیایی طیف تغییرات SiO2 در نمونه های آنالیزشده از 6/64 تا 4/74 متغیر است. الگوی تغییرات عناصر کمیاب بهنجار شده نسبت به گوشته اولیه و کندریت بیانگر غنی‌شدگی این سنگ‌ها از LILE و LREE و تهی شدگی از HFSE و HREE است. نسبت 85/0-28/0 Eu/Eu*= در نمونه های کلاه قاضی نشان‌دهنده حضور اندکی پلاژیوکلاز در باقی‌مانده ذوب در منبع است. بر اساس شواهد زمین‌شناسی، کانی شناسی و ژئوشیمیایی مجموعه نفوذی کلاه قاضی در ژوراسیک بالایی و در طی نفوذ مذاب‌های گوشته ای به داخل پوسته و ذوب‌بخشی سنگ‌های پوسته ای تشکیل شده است.

In the Ulten Zone (Upper Austroalpine), small bodies of mantle peridotites are incorporated within high-grade basement rocks (gneisses and migmatites) which represent remnants of lower crust subducted and reequilibrated at eclogite- facies conditions during the Variscan orogenic cycle. The Ulten peridotites record a complex metamorphic and deformative evolution, which is testified by the transition from coarse-grained protogranular spinel-bearing peridotites, to fine-grained garnet and amphibole (Ca-hornblende) -bearing peridotites with porphyroclastic to mosaic granoblastic textures. Thermometric estimates on the coarse-type spinel lherzolites have yielded high temperatures of equilibration, in the range 1100-1300°C (Obata and Morten, 1987). In the porphyroclastic peridotites, the metamorphic recrystallization to (garnet + amphibole)-facies conditions is evidenced by the development of: i) garnet coronas around spinel, ii) fine-grained granoblastic aggregates made by olivine + garnet + Ca-hornblende + new pyroxenes, iii) garnet and Ca-hornblende exsolutions within primary spinel-facies clino- and ortho-pyroxenes. The P-T conditions of the high-pressure eclogitic recrystallization which produced the spinel- to garnet-facies transition have been recently estimated to 850°C and 27 kbar (Nimis and Morten, 1999). The peculiar thermobarometric reequilibration recorded by the Ulten peridotites has been interpreted as the result of a wedge to slab evolution (Nimis and Morten, 1999; Godard et al., 1996). In this scenario, the spinel peridotites represent portions of a mantle wedge which were incorporated (by convection ) in a downgoing slab of cold continental crust, and were then subducted together with the slab to depths of about 90 km. Entrainment in the cold slab and subduction caused the reequilibration of the peridotites at 850°C and 27 kbar. The metamorphic transition from spinel- to garnetbearing assemblage occurred therefore in a dynamic regime, and was accompanied by significant input of metasomatic fluids, as testified by the crystallization of abundant amphibole in the garnet-bearing high-pressure assemblage. Petrologic investigations on the host gneissic basement rocks have evidenced that they also experienced high-pressure recrystallization, which was accompanied by in-situ partial melting and migmatization (Godard et al., 1996). The particular geodynamic evolution of the Ulten peridotites thus offer the unique opportunity to investigate the effects of crustal-derived metasomatism on mantle rocks involved in a subducting environment. Previous whole-rock chemical and isotopic investigations on the Ulten peridotites have evidenced that the fine-type garnet-facies ultramafics are enriched in LREE, K, Sr and that the alkalis enrichment is positively correlated with the 87Sr/86Sr ratios. In this study, we present the results of detailed in situ investigations (performed by the ion microprobe operating at CSCC, Pavia, Italy) on the trace element chemistry of the main mineral phases (clinopyroxene, amphibole and garnets) from seven selected samples representative of the various stages of the tectonometamorphic evolution recorded by the peridotites. Major aims have been to investigate the geochemical signature of fluids responsible of the amphibole crystallization, and provide further constraints on the nature of the metasomatic processes. The data obtained are potentially usefull to characterize, by direct evidence, the chemical changes induced in mantle rocks by crustal metasomatism. The coarse-type spinel peridotites, which are relics of the “pre-subduction”, mantle-wedge equilibration stage, display modest metasomatic effects. In these samples, modal metasomatism is only recorded by the incipient crystallization of amphibole as rims around clinopyroxene. Clinopyroxenes have almost flat REE spectra (CeN/SmN = 0.76-0.87) at 4-8 x C1 values, or display concave shape with selective LREE enrichment (CeN/SmN = 2.50-4.50, SmN/YbN = 0.53-0.97). The REE concentrations of amphiboles are very similar to those of clinopyroxenes. Both amphiboles and clinopyroxenes in sample MK5D, a coarse-type garnet-bearing peridotite, exhibit a convex-upward REE pattern characterized by LREE and HREE depletion (CeN/SmN = 0.17-0.19; SmN/YbN = 3.07-6.13). Their low HREE abundances are due to the equilibration with garnet which, as expected, show severely fractionated patterns (CeN/YbN < 0.001; HREE at about 20-30 x C1). Amphiboles, in the coarse-type rocks, also show low Sr (18-35 ppm) and K (171-964 ppm) abundances. The most evident metasomatic effects are recorded by the eclogite-facies recrystallized fine-type peridotites. In these rocks, modal metasomatism is documented by abundant crystallization of amphibole (Ca-hornblende, Mg values: 90- 92) in equilibrium with garnet. In some samples, the gnt+cpx+amph+opx+ol assemblage is replaced by amph+opx+ol assemblages, this feature indicating progressive degrees of hydration. Amphiboles display significant LREE enrichment (CeN/YbN = 3.90-11.50; LREE in the range 20-50 x C1) and high Sr (150-250 ppm), K (1910- 7280 ppm) and Ba (280-800 ppm) contents. By contrast, they have relatively low concentrations in HFSE (e.g., Zr = 14-25 ppm, Y = 6.7-16 ppm, Ti = 1150-2500 ppm, Nb = 2-7 ppm). The geochemical signature recorded by amphibole in the fine-type peridotites, i.e. the strong enrichment in LILE relative to HFSE, is a peculiar feature of crustal-derived metasomatic agents. The lack of evidence of major element modifications in mantle minerals (e.g. Mg-value decrese, crystallization of orthopyroxene around olivine) strongly suggest that the metasomatic agent was an hydrous fluid rather than a silica-rich melt. Moreover, experimental studies have demonstrated that aqueous fluids preferentially partition elements like alkalies, Ba, Sr and Pb, whereas they have scarce affinity for HFSE. The results of our study therefore indicate that the chemical modifications occurred in the Ulten peridotites during the high-pressure reequilibration were most likely produced by the input of hydrous, LILE-enriched, fluids, which caused crystallization of abundant amphibole. Such H2O-rich fluids could represent the residual fluids left after the crystallization of leucosomes, starting from water-undersaturated melts produced during migmatization of the host gneisses.

Dolerite dikes cut the Lower Paleozoic medium- to high-grade metamorphic basement, the Sardinia-Corsica batholith and the Stephanian - Autunian calcalkaline effusives (1st volcanic episode, according e.g. to Bonin, 1989; Cabanis et al., 1990) with a N-S trend and subvertical dip. Their occurrence is reported from several areas of Sardinia, from the N to the SE. The basic dikes are associated with diorite to rhyolite dikes having trends ranging from E-W to N-S. A late Triassic lamprophyric dike intruded the garnet-staurolite-kyanite micaschists near Mt. Nieddu and has a 40°N direction. Two 40Ar-39Ar age determinations for the dolerite dikes were carried out on amphibole. Ages between 253.8±4.9 and 248±8 Ma (Permian - Early Triassic) probably correspond to the emplacement interval. A continental within-plate geochemical signature, with prevalent tholeiitic to transitional (dolerites) and minor alkalic (lamprophyre) terms, has been determined. The relative LILE and REE enrichment compared with the MORBs and Nd isotopic ratios (4 bulk samples; initial 143Nd/144Nd ratios between 0.512529±0.000025 and 0.513143±0.000023) is consistent with a source in a sub-continental lithospheric mantle. The remarkable compositional and isotopic variability can be explained by the partial melting of a heterogeneous source. Crustal contamination variably affects some transitional dikes; the host Arzachena monzogranite probably represents the major contaminant source. On the whole, the geochemical data support an anorogenic geochemical affinity, in accordance with the extensional regional tectonics, for the Permo-Triassic dikes in Sardinia. The E-W emplacement patterns correspond to the major fractures and primarily R and RI structures associated with the late Variscan transcurrent-extensional E-W faults. The N-S emplacement trend is consistently associated with a generalized E-W extension of the Sardinian crust: in a predrift restoration it coincides with the patterns of extension between the European and Insubric crust at the beginning of the Tethyan rift. In Sardinia, the lamprophyre dike is significantly younger than the late Variscan alkaline volcanites and dikes of the rest of the Mediterranean domain (e.g. Eastern Provence: 278-264 Ma, 40Ar/39Ar isotope dating on feldspar from rhyolites and on plagioclase from mafic flow, Zheng et al., 1992; Western Pyrenean Axial Zone: 271-266 Ma, K-Ar isotope dating on kaersutite phenocrysts from mafic dikes, Debon and Zimmermann, 1993), in accordance with the diachronism of both the first and second volcanic episodes in different areas of the Mediterranean domain. The overlap between the calc-alkaline events and the volcanic and sub-volcanic alkalic event, is not exclusive to Sardinia and Corsica (e.g. the Pyrenees and Pan-African orogen).

Origin of S-type granites coeval with I-type granites in the Hellenic subduction system, Miocene of Naxos, Greece

Extension-related within-plate Cenozoic volcanism in the central-western Mediterranean mostly occurs in three provinces: Veneto, Adria plate, i.e. African northern promontory; Iblei (Sicily), African lithosphere; and Sardinia, a drifted fragment of the European lithosphere. In Veneto (Paleogene) and Iblei (Neogene-Quaternary), transtensional rift volcanism developed as foreland reaction to collisional processes along the Alpine and Maghrebian chains respectively, generating prevailing basic magmas - from tholeiites to Na-alkali basalts and nephelinites. In Sardinia, the Pliocene-Quaternary volcanism - related to general tensional tectonics of the central Mediterranean – produced comparatively more potassic magmas from subalkaline basalts, alkali basalts/trachybasalts to basanites, locally associated with rhyolitic and phonolitic differentiates. Integrated petrogenetic study, based on incompatible element and Sr-Nd-Pb isotope systematics for the three volcanic provinces, leads to the following constraints: 1) the primary magmas, from tholeiites, alkali basalts to basanites and nephelinites, were generated by decreasing melting degrees of progressively deeper lithospheric mantle sources (ca. 30 to 100 km depth); 2) extensive and multiple enrichment processes by alkali-silicate metasomatizing components widely affected all mantle sources, which invariably require 5 – 10% metasomatic volatile-bearing phases, mainly amphibole for Veneto and Iblei, and phlogopite + amphibole for Sardinia; 3) a previously depleted lithospheric mantle (DM) is enriched by prevailing HIMU (with subordinate EMII) geochemical components in both Veneto and Iblean Provinces, whereas in Sardinia distinctly more potassic EMI signature predominates, in addition to HIMU; 4) in all the investigated provinces, the more alkaline deeper basic magmas (compared to sub-alkaline basalts) show relatively more marked HIMU signatures, also with involvement of carbonatitic components in the lowest lithospheric sources where nephelinites were generated. Mantle xenoliths associated to alkaline lavas of the three volcanic provinces, generally represent shallow portions of the lithospheric mantle column (< 40-50 km depth, i.e. in the Mechanical Boundary Layer, MBL) according to thermobarometric estimates and rheologic characteristics (e.g. grain size). Petrological data invariably indicate that these mantle xenoliths underwent a complex compositional evolution, characterized by depletions due to extraction of basic magmas (mostly occurring in the pre-Palaeozoic), and multiple metasomatic enrichments reflected in compositional variations of the constituent mineral phases, particularly pyroxenes, and the variable occurrence of pyrometamorphic textures and new phases, including glass + feldspar + amphibole + phlogopite. The resulting isotopic signatures conform well to those of their host magmas, being dependent on the variable contributions of the DM, HIMU, and EM components: a prevailing HIMU imprint is recorded for both Veneto and Iblean mantle materials, whereas a predominant EMI component, in addition to HIMU, is observed for the Sardinian mantle. A comparison at a circum-Mediterranean scale reveals that the geochemical features of Sardinia are also observed in the Cenozoic anorogenic magmas and associated mantle xenoliths located within the Variscan basement of centralwestern Europe. Peridotite massifs, tectonically emplaced during the Alpine orogeny, are similarly characterized (in addition to HIMU) by a prevalent EMI signature, which could thus have been active at least since the pre-Mid Mesozoic. By contrast, EMI signature is lacking in Veneto and Iblei, as well as in the mantle xenoliths and host alkaline lavas from other occurrences of the North African domain, where the HIMU component predominates. The widespread presence of HIMU in the European/North African domains may be related to the existence of a Cenozoic plume region extending from the eastern Atlantic to Central Europe and Central-Western Mediterranean, also suggested by the seismic tomographic data. Accordingly, the more recent HIMU metasomatizing agents rising from the convecting mantle appear to have been effectively accumulated in the lower lithospheric portions (i.e. Thermal Boundary Layer, TBL), whereas older metasomatic components (e.g. EMI for Europe) may have been better preserved in the upper, more rigid lithospheric mantle (i.e. MBL).

Na região norte do Rio de Janeiro afloram rochas orto e paraderivadas neoproterozoicas de alto grau metamórfico, englobadas no Terreno Oriental da Faixa Ribeira. Dado a carência de estudos de detalhe na região, o presente trabalho contribui com novos aspectos de campo, petrografia, química mineral e geotermobarometria dos granitoides diatexíticos aflorantes na região entre São José de Ubá e São João do Paraíso, no noroeste fluminense. O litotipo é caracterizado por biotita-hornblenda gnaisses migmatíticos e protomiloníticos que são correlacionáveis a ortognaisses e granulitos do Complexo Serra da Bolívia (Domínio Cambuci da Faixa Ribeira). Dois litotipos neoformados são observados: um mesocrático, de caráter residual; e outro leucocrático, que correspondente ao leucossoma. O enriquecimento de K-feldspato no leucossoma em relação ao neossoma residual sugere baixos volumes de fusão por meio de reações de fusão envolvendo água. Aplicando-se diferentes geotermobarômetros, foram obtidas como condições de equilíbrio para o pico metamórfico 6,8 Kbar e 672–680°C (fácies anfibolito superior). Tais valores destoam dos encontrados na literatura para ouras rochas da região, reforçando a ideia de que os domínios do Terreno Oriental da Faixa Ribeira apresentam evoluções metamórficas distintas, em diferentes níveis crustais.

The Limpopo Belt of southern Africa is generally believed to represent the root of a late Archaean continental collision zone, and has been used to demonstrate the validity of the uniformitarian concept in tectonics Large scale tectonic models have been applied in spite of the fact that large portions of the belt yet await the most basic investigations. Here we report the first detailed field and petrographic study of the northernmost part of the Limpopo Belt, the Northern Marginal Zone sensu stricto (NMZ s.s) and conclude, on the basis of relative age relation, nature of PT evolution and deformation, that none of the current models can correctly explain the evolution of the study area. The evolution of NMZ s.s, is complex and includes four metamorphic stages, two major plutonic episodes and at least three deformation events. The oldest rocks, mafic granulites, record all four stages of metamorphic mineral growth. The first two stages predate deposition of sediments and the intrusion of voluminous enderbite and charnockite between 2.72 and 2.62 Ga. The bulk of our observations relate to the late Archaean (stage 3) granulite facies event. Abundant reaction textures are preserved in mafic granulite, metasediments, metamorphosed charnockite and enderbite and allow us to qualitatively reconstruct the PT evolution. Prograde heating occurred in the sillimanite stability held. During peak temperature conditions vapour-absent melting is observed in most felsic lithologies,Temperatures between 800 and 850 degrees C at pressures as low as 0.4-0.5 GPa are indicated by various mineral assemblages. The thermal peak was followed by an increase in pressure. Typical reactions of anti-clockwise PT evolution, like breakdown of cordierite + spinel to sapphirine and orthopyroxene + plagioclase to garnet + quartz, are frequently found. Maximum pressure is constraint to ca. 0.85 GPa by the complete absence of garnet in mafic granulites. Initial cooling was rapid, and is indicated by the back reaction of melt + orthopyroxene to biotite + quartz symplectites preserved in migmatites. This metamorphic event was accompanied by intrusion of porphyritic charnockite and granite, and by coeval compressional deformation. The observed evolution, especially the combination of an anti-clockwise PT loop and compressional tectonics, requires a strong, transient heat-source affecting the base of the crust. Neither the thermal evolution nor the relative timing is correctly predicted by existing collision models. The NMZ s.s. granulites were finally exhumed in a separate event along upper greenschist-facies thrusts, in response to a transpressive orogeny affecting the units further south at 2.0 Ga. In spite of potential ambiguities inherent to a qualitative approach, our observations show that petrography and field work, if used in conjunction with dating of a few key age relations and structural interpretation, are a pre-requisite to the erection of realistic tectonic models. The example of the NMZ s.s. may encourage geoscientists with limited access to analytical facilities to reassess the geological evolution of terrains which lack basic description.

The Hohonu Batholith is an aggregation of mostly mid-Cretaceous granitoid plutons on the West Coast of the South Island of New Zealand emplaced during a transitional period between subduction-related compression and continental lithospheric extension. This study reports an integrated dataset, comprising in-situ UPb, O and Hf isotope compositions and REE, Ti and other trace and major elements (ZrHf, ThU) for zircons extracted from four representative plutons within the batholith. Our results provide detailed insight into the protracted thermal, chronological and geochemical histories. LA-ICPMS UPb zircon ages indicate a primary episode of magma genesis and emplacement from 107 to 113 Ma, confirming published SIMS dating. However, a younger previously unrecognized age population of ~91–96 Ma is identified, primarily (although not exclusively) in zircon rims. This younger age event coincides with the timing of protracted lithospheric extension and crustal thinning of the Zealandia continent. The cryptic younger zircon ages suggest that Hohonu granitoids experienced a partial thermal overprint (accompanied by Pb loss) mostly recorded in rims. Differences in bulk rock geochemistry between plutons are inferred to reflect variable conditions of partial melting controlled by source mineralogy and H2O content. Isotope and trace element compositions, along with Ti-thermometry, measured on the same micro-volume of CL-imaged zircons, are used to test if source characteristics were imparted from melt to minerals in zircon-saturated silicic systems. Similarities are revealed in the zircon record of the selected plutonic rocks, confirming their broadly consanguineous relationship and the fundamental role of open-system behaviour, involving hybridization or assimilation between mantle-derived (or juvenile mafic) and a crustal-derived components, as previously inferred from whole rock NdSr isotope systematics. However, intra-sample decoupling of zircon OHf isotope systematics may also be linked to residual source component unmixing. This possibility, in addition to mafic recharge, may have obscured melt source compositional characteristics, and hence zircon REE appear as unsuitable fingerprints of source(s) and conditions of partial melting in this granitoid system. Simple compositional and thermal magma evolution trends appear punctuated by episodes of mafic recharge, presumably during lithospheric thinning.

Small peridotite and gabbro bodies outcrop within medium- high grade, Grt- and Sil-bearing granulitic and migmatitic rocks of the Calabrian Thyrrenian Coastal Chain, northern sector of the Calabrian-Peloritan Arc, southern Italy. These bodies generally have sharp contacts with the country rocks. The peridotites, which sometimes include folded, transposed and boudinated pyroxenite veins and/or gabbroic dykes, are heavily serpentinized, although small, less serpentinized portions are locally preserved. These fresher portions show porphyroclastic microstructure, with olivine (Fo 90-93), orthopyroxene (En 89-91; Al2O3 = 1.1 - 2.9 wt%), brown spinel (Cr/(Cr+Al) 0.07-0.75), very rare clinopyroxene (Ca/(Ca+Mg) = 0.51-0.52; Al2O3 = 2.36-3.22 wt%), amphibole (Mg-hornblende - pargasite), chlorite, antigorite, lizardite 1T and magnetite. The pyroxenite veins are represented by abundant orthopyroxenites, Amph-pyroxenites and scarce Spl-websterites. The orthopyroxenites have porphyroclastic to granoblastic polygonal microstructures, with porphyroclasts of orthopyroxene (En 88-89; Al2O3 = 3.06-3.92 wt%), olivine (Fo 90) and hercynitic spinel set up in a well-recrystallized, polygonal matrix of orthopyroxene (En 88; Al2O3 = 3.5 wt%), olivine (Fo 90), amphibole (Mg - hornblende), serpentine, chlorite and diaspore. The Amph-pyroxenites show porphyroclastic texture with large porphyroclasts of amphibole (Mg-hornblende) and clinopyroxene (Ca/(Ca+Mg) = 0.49-0.51; Al2O3 = 1.9- 2.1 wt%) set up in a well-recrystallized matrix of amphibole (pargasite to Mg-hornblende), clinopyroxene (Ca/(Ca+Mg) 0.58; Al2O3 = 1.94-2.1 wt%), orthopyroxene (En 86-87; Al2O3 = 2.75-3.05 wt%), hercynitic spinel (Cr/(Cr+Al) = 0.09-0.13) and magnetite. The Spl-websterites have a porphyroclastic microstructure, with clinopyroxene (Ca/(Ca+Mg) 0.48; Al2O3 = 0.6 wt%), orthopyroxene (En 89; Al2O3 = 0.6 wt%) and brown spinel (Cr/(Cr+Al) 0.54). The serpentinized peridotites have MgO/FeOt values ranging from 5.1 to 7.6, which coupled with their Ni-Co-Sc patterns suggest a residual character. The REE patterns, in spite of some zig-zag behaviour of L - and MREE, possibly due to the serpentinization process, are also indicative of a depleted character (CeN/YbN = 0.03-0.65). The metagabbros show a tholeiitic fractionation trend. Incompatible elements spider diagrams normalized against MORB show more or less flat patterns around 1, except for a conspicuous negative Th anomaly. The REE normalized patterns show either a positive slope from La to Eu followed by a more or less flat trend at about 10 x Ch, or almost flat trend at around 10 x Ch. Some samples show slight, positive Eu anomaly. The LaN/YbN ratios range from 0.07 to 2.39. Their geochemical signature would be consistent with an underplating magmatism. The REE abundances of the pyroxenites, as well as their Ni-Co-Sc patterns, suggest a derivation from reacting melts and/or fluids. Temperatures of equilibration of about 800°C and 750°C have been calculated for the peridotites - pyroxenites and metagabbros, respectively. These data fit quite well the inferred physical conditions (750-800°C and about 0.9 - 1.1 GPa) for the metamorphic climax of the granulitic country rocks (Piluso et al., 1998). Preliminary barometric data are also consistent with these estimates. From the above considerations and in the light of the evolution model proposed for the northern sector of the Calabrian Arc by Piluso and Morten (1999), we suggest that: i) astenophere upwelling, responsible for the thermal anomaly, melting, and granulitic metamorphism in the crustal rocks, produced lithosphere thinning (after Variscan orogenic collapse) and underplating gabbroic magmatism in the northern sector of the Calabrian Arc; ii) rising melts/fluids formed pyroxenitic and gabbroic dykes in the underlying mantle peridotites, which were coupled with the crustal rocks before or during the migmatitic-granulitic event dated at 295 Ma (Shenk, 1989); iii) the peridotitespyroxenites- gabbros and the crustal rocks subsequently shared in a common exhumation history through HT and LT detachment shear zones (Piluso and Morten, 1999), under high amphibolite, amphibolite and greenschists facies conditions.