Hercynian Granodiorite Research Articles

Geochronology, petrogenesis and metallogenic implications of granitoids in the Xiaotuergen Cu deposit, Northern Chinese Altai Orogen, NW China: Constraints from zircon, apatite and whole-rock geochemistry

The Xiaotuergen Cu deposit is the first Cu deposit described in the Northern Chinese Altai Orogen, Xinjiang, NW China. The multiple phases of granitic intrusions occur in the Xiaotuergen region, including the intrusions of granodiorite porphyry, granite porphyry, and biotite monzogranite. Among these types of intrusions, Cu mineralization is considered to be closely related to the early Hercynian granodiorite porphyry. However, the precise emplacement ages, petrogenesis, redox states, and tectonic setting of the three Xiaotuergen granitoids and the relationship between these granitoids and mineralization was unclear. To resolve these issues, zircon U–Pb dates, zircon in situ trace element concentrations, Hf isotopic compositions, apatite in situ major element concentrations, and whole-rock geochemistry were determined in this study. Three different intrusive phases are recognized within the district; in order of deceasing age, these phases are biotite monzogranite, granodiorite porphyry, and granite porphyry, which have the ages of 400.2 ± 1.6 Ma to 399.6 ± 2.4 Ma, 390.3 ± 1.6 Ma to 377.1 ± 1.6 Ma, 381.0 ± 2.0 Ma to 378.7 ± 1.2 Ma, respectively, suggesting that they were emplaced during the Early-Middle Devonian. Geochemical data show that these intrusions are metaluminous to weakly peraluminous (A/CNK = 0.91–1.28), calc-alkalic to shoshonitic I-type granitoids. The two Harker plot trends reveal derivation from two magma sources (biotite monzogranite vs granodiorite porphyry and granite porphyry) that experienced different evolutionary processes. Multiple indicators, including the zircon Ce4+/Ce3+ ratio and oxygen fugacity (logfO2) values, apatite Mn oxybarometer, and whole-rock Fe2O3/FeO ratio, consistently indicate that the parental magma of the fertile granodiorite porphyry was more oxidized than that of the two barren intrusions (biotite monzogranite and granite porphyry). Furthermore, apatite grains from the fertile granodiorite porphyry have higher Cl contents and Cl/F ratios than those of the ore-barren biotite monzogranite and granite porphyry, which may have resulted from the higher water content of the parental magma and increased lithospheric mantle-derived fluid involvement in their generation. Thus, higher Cl/F ratios of apatite can be used to discriminate fertile granitoid intrusions that host Cu mineralization in the Northern Altai orogen. Zircon Hf isotopic compositions show widely ranging and mostly positive εHf (t) values (−4.2 to +11.4, mainly concentrated between +2 and +8). These isotope characteristics in combination with young zircon TDMC (Hf) ages (655–1,659 Ma; mainly concentrated between 850 Ma and 1250 Ma) indicate that the Xiaotuergen granitoids were most likely derived from the partial melting of juvenile lower crust, with some interaction of mantle materials from the underlying mantle wedge. The three Xiaotuergen granitoids were enriched in light rare earth elements and large ion lithophile elements, depleted in heavy rare earth elements, and showed negative Nb, Ta, Sr, P, and Ti anomalies, displaying typical arc geochemical affinities. Combined with the regional tectonic evolution, we propose that the Xiaotuergen granitoids were generated in a continental arc setting associated with subduction processes, which were the products of syn-accretion orogenic processes.

Strontium isotopes in high- and low-Ge coals from the Shengli Coalfield, Inner Mongolia, northern China: New indicators for Ge source

The coal-hosted Ge deposit (Early Cretaceous) at Wulantuga, Inner Mongolia, northern China, is one of the largest Ge deposits in the world. Although the origin, modes of occurrence, and spatial distribution of Ge in this deposit have been investigated by several studies, the provenance and pathways of Ge from the source to the deposit are largely unknown. In this study, we investigated the 87Sr/86Sr values in the high-Ge coal of the Wulantuga deposit and the stratigraphically-equivalent low-Ge coal of the No. 1 Pit, both from the Shengli Coalfield, Inner Mongolia, northern China. In addition to an organic association, Sr and Ca in the high-Ge and low-Ge coals also occur in gypsum. The water-soluble Sr (i.e., in gypsum) in the high-Ge coal (Ge concentrations: 45–1170 ppm, 273 ppm on average, whole coal basis) has 87Sr/86Sr values in the range of 0.710411 to 0.710633 (with an average of 0.710478). In contrast, the low-Ge coal (Ge concentrations: up to 4.23 ppm, 0.45 ppm on average, whole coal basis) has lower 87Sr/86Sr values in two sub-groups, a low range of 0.709896–0.71005 (with an average of 0.709736) and a high range of 0.710173–0.710547 (with an average of 0.710329 and dominantly in the range of 0.710173–0.710406). The higher 87Sr/86Sr values in the high-Ge coals indicate that the highly-elevated concentration of Ge is associated with a local source of radiogenic Sr. The enrichment of Ge in the coal is due to several consecutive processes. Surface waters penetrated deeply into granite and dissolved at least partially more soluble phases of the granite. The Ge-rich solution flowed over ground or underground into the Shengli Coalfield and Ge precipitated in the peat, the precursor of the coal investigated in the coalfield. The 87Sr/86Sr values of the water-soluble fraction, the elemental compositions of high-Ge and low-Ge coals, and the geomorphological setting of the coalfield point to the Hercynian granodiorite intrusion on the nearby SE margin of the Shengli Coalfield as the primary source of Ge in the Wulantuga coal-hosted Ge deposit. In addition to serving as an indicator of the mode of formation of the coal-hosted Ge deposit, the 87Sr/86Sr values and their spatial distribution can provide an indication for the hydrological pathways of the mineralising fluid.

Genesis of the Tianbaoshan Pb–Zn–Cu–Mo polymetallic deposit in eastern Jilin, NE China: Constraints from fluid inclusions and C–H–O–S–Pb isotope systematics

The Tianbaoshan Pb–Zn–Cu–Mo deposit is located in the eastern part of Jilin–Heilongjiang region, NE China which is considered to be the eastern segment of the Central Asian Orogenic Belt. Field and geochronological evidences indicate that this deposit experienced three types of mineralization including Hercynian skarn, cryptoexplosion breccia pipe, and Yanshanian quartz vein. The early stages of the Pb–Zn–Cu mineral systems in skarn and cryptoexplosion breccia pipe are characterized by a high-temperature, high-salinity H2O–CO2–NaCl system of hydrothermal fluids that were possibly exsolved from the Hercynian wall-rock granodiorite. These fluids show H–O isotopic compositions similar to those of typical magmatic fluids. By contrast, the low-temperature hydrothermal fluids of the later stages are represented by low-salinity NaCl–H2O systems with low H–O isotopic values. The skarn and cryptoexplosive breccia pipe types of Pb–Zn–Cu mineralization tend to have weakly negative δ34S values of −4.0‰ to −0.8‰ (mean values of −2.31‰ and −2.16‰, respectively), indicating that the sulfur was sourced from the Hercynian magma. Therefore, the early stage ore-forming fluids of the skarn and cryptoexplosive breccia pipe were most likely sourced from high-temperature and high-salinity fluids closely related to the cooling and fractional crystallization of the Hercynian granodiorite, while the later stages changed to NaCl–H2O meteoric water influx. Whereas the ore-forming fluids of the quartz vein type of Mo mineralization were high-temperature, high-salinity NaCl–H2O systems that differed from those of the skarn and cryptoexplosion breccia pipe, but their H–O isotopic compositions also indicate a magmatic fluid. The weakly enriched δ34S values of molybdenite from the quartz vein type Mo mineralization (δ34S=0.2–2.8‰, average of 1.65‰) are comparable with those of other Mesozoic Yanshanian Mo deposits (δ34S=0.4‰–4.1‰, with an average of 1.39‰–3.15‰), but differ significantly from those of the Hercynian Pb–Zn–Cu skarn and cryptoexplosion breccia pipe. This indicates that the sulfur of quartz vein type of Mo originated from Mesozoic Yanshanian magmatism and that the ore-forming fluids were derived from Yanshanian magmatic rocks rather than being a product of the Hercynian activity. The δ13C values of the fluid inclusions in quartz from the skarn, cryptoexplosion breccia pipe and quartz vein types are in a narrow range of −19.5‰ to −9.3‰, similar to those of the Shanxiuling Group, which indicates that the carbon of the three types of mineralization had the same primary origin in the Shanxiuling Group. The lead isotope compositions of ores from the skarn and cryptoexplosion breccia pipe types of mineralization (206Pb/204Pb=18.0725–18.3627, 207Pb/204Pb=15.3721–15.5694 and 208Pb/204Pb=37.5542–38.8208) overlap with those of the Hercynian granodiorite and Shanxiuling Group marble, suggesting that the lead was probably derived from a mix of two different sources, the Shanxiuling Group and the Hercynian granodiorite. Whereas the lead isotope compositions of ores from the quartz vein type of Mo mineralization (206Pb/204Pb=18.3837–18.6949, 207Pb/204Pb=15.6824–15.7293 and 208Pb/204Pb=39.1009–39.1889) are significantly higher than those of the Shanxiuling Group marble and the Hercynian granodiorite. This indicates that the lead may be a product of Yanshanian magmatic activity instead of the nearby Hercynian granodiorite.

Archaeopetrological approximation to the lithic procurement of the neolithic axes and adzes from Can Sadurní’s cave (Begues, NE Iberian Peninsula)

Can Sadurní’s cave, located in Begues (NE Iberian Peninsula), in the Baix Llobregat region, is an archaeological site with a wide stratigraphic sequence covering from the Epipaleolithic hunter-gatherers societies at the beginning of the Holocene to Roman times. During the excavations of the last years a large number of different raw materials used for the manufacture of axes have been recovered.The present study is focused on the Neolithic sequence. 31 axes and adzes have been characterised petrographically with the aid of a binocular microscope and transmission microscope. A great variety of rocks constitute the raw materials of these stone tools, ranging from contact and regional metamorphic rocks to plutonic and porphyric igneous rocks. The formers are the most abundant (up to 78%) and include hornfels, spotted phyllites, marbles, quartzites, slates and phyllites. The igneous rocks consist of granodiorite, porphyry and aplites. Such a great assemblage of rocks matches in a geological context representative of a plutonic intrusion and its metamorphic contact aureole. Following that scenario we suggest that the most likely source area for all these materials occur at the Collserola hills, at 27 km far to the east from the cave, at the other side of the Llobregat River, where an Hercynian granodiorite and related igneous rocks intruding Ordovician metasedimentary materials are presented.

Mantle and crustal processes in the magmatism of the Campania region: inferences from mineralogy, geochemistry, and Sr–Nd–O isotopes of young hybrid volcanics of the Ischia island (South Italy)

Ischia, one active volcano of the Phlegraean Volcanic District, prone to very high risk, is dominated by a caldera formed 55 ka BP, followed by resurgence of the collapsed area. Over the past 3 ka, the activity extruded evolved potassic magmas; only a few low-energy explosive events were fed by less evolved magmas. A geochemical and Sr–Nd–O isotope investigation has been performed on minerals and glass from products of three of such eruptions, Molara, Vateliero, and Cava Nocelle (<2.6 ka BP). Data document strong mineralogical, geochemical, and isotopic heterogeneities likely resulting from mingling/mixing processes among mafic and felsic magmas that already fed the Ischia volcanism in the past. Detailed study on the most mafic magma has permitted to investigate its origin. The mantle sector below Ischia underwent subduction processes that modified its pristine chemical, isotopic, and redox conditions by addition of ≤1 % of sediment fluids/melts. Similar processes occurred from Southeast to Northwest along the Apennine compressive margin, with addition of up to 2.5 % of sediment-derived material. This is shown by volcanics with poorly variable, typical δ18O mantle values, and 87Sr/86Sr progressively increasing toward typical continental crust values. Multiple partial melting of this modified mantle generated distinct primary magmas that occasionally assimilated continental crust, acquiring more 18O than 87Sr. At Ischia, 7 % of Hercynian granodiorite assimilation produced isotopically distinct, K-basaltic to latitic magmas. A SW–NE regional tectonic structure gave these magmas coming from large depth the opportunity to mingle/mix with felsic magmas stagnating in shallower reservoirs, eventually triggering explosive eruptions.

Importance of porosity and transfer of matter in the rock weathering processes: two examples in central Spain

Some physical properties (bulk and free porosity, pore size distribution), and the chemical compo- sition and mass balance of two deeply weathered profiles one developed on Hercynian granodiorite and the other on pre-Cambrian slates were studied. Hydric and mercury porosimetry, nitrogen adsorption techniques, chemical analyses and XRD techniques were used. On granodiorite, weathering has created increased porosity with a pore diameter \5 lm, whereas on slates the weathering has produced of ca. 1 lm in diameter. These pore sizes have played an important role in the weathering processes. Assuming that weathering preserves volumes, except in the uppermost part of the profiles, it brought about a loss of matter of more than 12% (*300 kg/m 3 ) on granodiorite and ca. 30% (*800 kg/m 3 ) on slates. These changes are related to shifts in the mineralogical evolution, with the appearance of new 2:1 and 1:1 phyllosilicates and Fe oxy- hydroxides as the main authigenic minerals. The release of matter, at least since the upper Neogene until the present, has led to the lowering of relief in a more or less homo- geneous way, giving rise to gentle hillsides and flat sur- faces below which the current river networks are incised. Porosity studies have the potential to explain several specific landforms as well as affecting landscape devel- opment in general.

Rare earth elements in waters from the albitite-bearing granodiorites of Central Sardinia, Italy

With the aim of contributing to the knowledge of the geochemical behaviour and mobility of the rare earth element (REE) in the natural water systems, the ground and surface waters of the Ottana–Orani area (Central Sardinia, Italy) were sampled. The study area consists of albititic bodies included in Hercynian granodiorites. The waters have pH in the range of 6.0–8.6, total dissolved solid (TDS) of between 0.1 and 0.6 g/l, and major cation composition dominated by Ca and Na, whereas predominant anions are Cl and/or HCO 3. The pH and the major-element composition of the waters are the factors affecting the concentration of REE in solution. The concentrations of ∑REE+Y in the samples filtered at 0.4 μm vary between 140 and 1600 ng/l, with La of between 14 and 314 ng/l, and Yb of between <6 and 12 ng/l. A negative Ce anomaly, especially marked at high pH, is observed in the groundwaters. The surface waters show lower REE concentrations, which are independent of pH, and negligible Ce anomaly. Speciation calculations, carried out with the EQ3NR computer program, showed that the complexes with the CO 3 2− ligand are the dominant REE species at pH in the range of 6.7–8.6. The REE 3+ ions dominate the speciation at pH <6.7 and only in the light REE (LREE). The relative concentrations of REE in water roughly reflect those in the aquifer host rocks. However, when concentrations of REE in water are normalised relative to the parent rocks, a preferential fractionation of heavy REE (HREE) into the water phase can be observed, suggesting the greater mobility and stability of HREE in aqueous solution.

Knowledge of the three French underground laboratory sites

From March 1994 to August 1996, ANDRA conducted a large reconnaissance work on three sites for possible underground research laboratories. These surveys highlighted for each site the presence of a geological formation, the properties of which meet the requirements of the French Fundamental Safety Rules. On the East site, the selected formation is a 130 m thick argillite layer. On the Gard site the layer is composed of close-packed siltites which are 200–400 m thick. On the Vienne site a hercynian granodiorite batholith was found under a 160 m thick jurassic overdurden.

Large-Scale Hydrothermal Processes in the Hercynian Massives of Eastern Pyrenees (France)

Metasomatic processes, resulting in massive feldspar rocks (either albite, oligoclase or K-feldspar) or muscovite+biotite rocks, have affected several palaeozoic massives in Eastern Pyrrnres, including mainly: (1) the Agly massif, composed of metasedimentary (granulite facies) precambrian gneisses overlain by mesoand epizonal schists of ordovician to silurian age; hercynian granodiorites are intrusive in these formations (2) the Millas massif, situated south of the Agly massif from which it is separated by the North-Pyrenean fault, and composed of hercynian granodiorite intruding a series of metapelites, partly preserved as schlieren in the southermost part of the massive. All granitoids exhibit at some places numerous dioritic to gabbroic enclaves, up to several hundred meters large.

REE-bearing minerals in the albitites of central Sardinia, Italy

AbstractRecent studies on albitite rocks located in the granodiorite complex of Central Sardinia have revealed that epidote has a widespread occurrence as a light rare-earth element (LREE)-bearing accessory common phase. Titanite has been recorded as a heavy rare earth element (HREE)-bearing mineral. The Hercynian granodiorite complex of Central Sardinia is composed chiefly of quartz, Ca-plagioclase, K-feldspar and biotite and of a wide variety of secondary assemblages, mainly allanite, titanite and zircon. Albitic plagioclase and quartz are the main mineral components of the albitites. Additional minerals include, besides allanite and epidote, a more calcic-plagioclase (oligoclase), K-feldspar, chlorite, titanite and more rarely muscovite. The mineral assemblages and REE-bearing minerals of albitites were analysed by wavelength dispersive spectrometry (WDS). Chemical data suggest that there is a near complete solid-solution between epidote and allanite whereas little variations in HREE of titanites were detected. In epidote-group minerals a pronounced zoning in REE was observed while titanite was recorded unzoned. Textural relations were studied by SEM to distinguish primary from secondary epidotes. Chemical criteria to recognize magmatic from alteration epidotes were also applied. The alteration epidotes mainly occur and generally originate from plagioclase alteration and from leaching of magmatic allanite. Comparison of textures using both the SEM technique and EPMA data showed that the characteristic ‘patchy zoning’, observed in epidotes, corresponds with different amounts of REE in these minerals.The schematic model proposed for the epidote-forming reactions during the metasomatic processes that affected the granodiorites involves: (i) the instability of the anorthitic component of plagioclase; (ii) the simultaneous formation of albite; (iii) the leaching of the magmatic allanite with a redistribution of REE in the epidotes of the albitites.

Cierco Pb-Zn-Ag vein deposits; isotopic and fluid inclusion evidence for formation during the Mesozoic extension in the Pyrenees of Spain

The Cierco Pb-Zn-Ag vein deposits, located in the central Pyrenees of Spain, crosscut Paleozoic metasedimentary rocks and are in close proximity to Hercynian granodiorite dikes and plutons. Galena and sphalerite in the deposits have average delta 34 S values of - 4.3 and - 0.8 per mil (CDT), respectively. Coexisting mineral pairs give an isotopic equilibration temperature range of 89 degrees to 163 degrees C which overlaps with the 112 degrees to 198 degrees C range obtained from primary fluid inclusions. Coexisting quartz has a delta 18 O value of 19 + or - 1 per mil (VSMOW). The fluid which deposited these minerals is inferred to have had delta 18 O (sub H 2 ) O and delta 34 S (sub H 2 ) S values of 5 + or - 1 and -1 + or - 1 per mil, respectively. Chemical and microthermometric analyses of fluid inclusions in quartz and sphalerite indicate salinities of 3 to 29 wt percent NaCl equiv with Na (super +) and Ca (super 2+) as the dominant cations in solution. The Br/Cl and I/Cl ratios differ from those characteristic of magmatic waters and pristine seawater, but show some similarity to those observed in deep ground waters in crystalline terranes, basinal brines, and evaporated seawater. Barite, which postdates the sulfides, spans isotopic ranges of 13 to 21 per mil, 10 to 15 per mil, and 0.7109 to 0.7123 for delta 34 S, delta 18 O, and 87 Sr/ 86 Sr, respectively. The three parameters are correlated providing strong evidence that the barites are products of fluid mixing.We propose that the Cierco deposits formed along an extensional fault system at the margin of a marine basin during the breakup of Pangea at some time between the Early Triassic and Early Cretaceous. Sulfide deposition corresponded to an upwelling of hydrothermal fluid from the Paleozoic basement and was limited by the amount of metals carried by the fluid. Barite deposition corresponded to the waning of upward flow and the collapse of sulfate-rich surface waters onto the retreating hydrothermal plume. Calcite precipitated late in the paragenesis as meteoric or marine waters descended into the fault system, possibly during a regression in the overlying basin.There are other deposits resembling Cierco elsewhere in the Iberian peninsula. Taken as a group, they are evidence that hydrothermal circulation systems were widespread during Mesozoic extension. Differences among the deposits can be related to the fact that H 2 S and other solutes had local and variable sources.

Relationships between intermediate and acidic rocks in orogenic granitoid suites: petrological, geochemical and isotopic (Sr, Nd, Pb) data from Capo Vaticano (southern Calabria, Italy)

Major, trace element and isotopic data are reported for Hercynian granodiorites and tonalites from Capo Vaticano, Calabria, with the aim of clarifying the relationships between acidic and intermediate lithologies in orogenic intrusive suites. These granodiorites (SiO 2 = 70.5%–73.5%) are peraluminous, relatively depleted in Rb, U, Th, and Pb, and display a large variation in many geochemical and isotopic parameters. REE show fractionated, cross-cutting chondrite-normalized patterns with small but significant negative Eu anomalies. Initial (290 Ma) Sr and Nd, and present-day Pb isotope ratios are: ( 87 Sr 86 Sr ) 290 = 0.7105–0.7110 ; ( 143 Nd 144 Nd ) 290 = 0.51175–0.51221 ; 206 Pb 204 Pb = 18.35–18.50 ; 207 Pb 204 Pb = 15.64–15.76 ; 208 Pb 204 Pb = 38.51–39.03 . Pb isotope ratios determined for separated feldspars show similar 207 Pb 204 Pb and 208 Pb 204 Pb and higher 207 Pb 204 Pb ratios compared to the present-day values for the whole rock. The tonalites (SiO 2 = 57.6%–67.3%) are typically calc-alkaline in composition and most of them are weakly peraluminous. They display large heterogeneities in trace-element abundances, REE patterns and Nd and Pb isotope ratios, both between and within individual intrusive units. Sr, Nd and Pb isotopic compositions display a range of values close to that of the associated granodiorites ( 87 Sr 86 Sr ) 290 = 0.7099–0.7111 ; ( 143 Nd 144 Nd ) 290 = 0.51185–0.51224 ; 206 Pb 204 Pb = 18.31–18.56 ; 207 Pb 204 Pb = 15.65–15.72 ; 208 Pb 204 Pb = 38.41–39.59 . The obtained data indicate that the tonalites and granodiorites share many compositional features which point to a genetic linkage. Likewise, many petrological, geochemical and field data exclude the possibility that all the granodiorites derive from intermediate magmas by any common evolutionary process, such as crystal-liquid fractionation or assimilation-fractional crystallization (AFC), and point to a different genesis for the two rock types. An origin of the granodiorites by the mixing of two separate acidic melts, one of crustal anatectic origin and one probably derived from an intermediate magma by crystal/liquid fractionation or AFC, is suggested by the data presented. The tonalites appear to have been generated by interaction of a mafic magma with crustal end-member(s) with isotopic and geochemical signatures similar to those of the granodiorites. The large involvement of crustal material in the genesis of the Capo Vaticano granitoids is responsible for the many common compositional characteristics observed in the intermediate and acidic rocks. However, a contribution from mantle components is also indicated by petrological and geochemical data, as well as by the occurrence of mafic enclaves commonly present in the tonalites. Nevertheless, the geochemical and isotopic features of such a mantle component appear to be greatly diluted by a large quantity of crustal material involved in the genesis of the studied granitoids.

The significance of garnet in the Permo-Carboniferous volcanic rocks of the Pyrenees

Garnet occurs within rhyolitic to dacitic lavas, ignimbrites and volcaniclastic sediments of the Pyrenean calc-alkaline Permo-Carboniferous volcanic rocks. On the basis of texture and major and trace element composition, the garnet is interpreted as having a phenocrystal as opposed to a xenocrystal or restite origin. The Pyrenean silicic volcanics are strongly peraluminous, consistent with derivation as a partial melt from a sedimentary source. They were erupted contemporaneously with the Hercynian thermal anomaly, which produced regional granulite facies metamorphism and anatectic granites. The ‘Late Hercynian granodiorites’ were also intruded at about this time. Geochemical data suggest that the petrogenesis of the volcanic rocks differed from that of the granodiorites. REE garnet/matrix partition coefficients for garnets from a welded rhyolitic ignimbrite are similar to those reported for other phenocrystal garnets in rhyolites. REE abundances imply that the garnets crystallized from a HREE depleted melt and this is considered to reflect the effects of garnet in the restite. Experimental constraints suggest that the garnet crystallized at pressures &gt;4–5kb, implying that the rhyolites originated from &gt; 14–17km depth and were erupted rapidly from these depths in order for the garnet to survive. These figures place constraints on the thermal regime prevailing in the late stages of the Hercynian orogeny.

Chilled margins and commingling of magmas in the Bressanone (Brixen) Hercynian granodiorites (Eastern Alps, northern Italy)

The Brixen Hercynian pluton is the result of a multiple intrusion of crustal anatectic magma (granodiorite-granites and two-mica cordierite-bearing leucogranites) and subcrustal magma (gabbro-diorites and fayalite-bearing leucogranites). Some adjacent granodioritic stocks show chilled margins containing globular inclusions in the size of several centimetres of a coarse-grained leucogranite. The granodiorite margins also show narrow zoning with more acidic external borders. This phenomenon is correlated to differentiated processes such as filter pressing and the Soret effect, which were active during the rise of these stocks. The strong petrographic and chemical analogies between the globular inclusions of (some) chilled margins and the fayalite leucogranites suggest their common subcrustal origin. The granodiorite containing these inclusions is of crustal origin: the occurrence of globular inclusions inside it thus indicates only mechanical mingling between crustal anatectic and subcrustal magmas, as shown by the absence of hybrids and chemical interactions.

Magma mixing: origin of intermediate rocks and “enclaves” from volcanism to plutonism

Intermediate magmatic rocks are rarely homogeneous; a common example is the presence of magmatic “enclaves” Volcanic examples show that they are indications of magma mixing processes. Mixing events in the trachyandesitic suites from the Sancy Volcano (France). The volcanic activity from Sancy consists of several brief trachyandesitic cycles. Each of them begins with the eruption of highly heterogeneous benmoreites, followed by more homogeneous mugearites. Light porphyritic trachyandesites enclose numerous inclusions of varied darker lavas. The crenulated geometry of the contacts, the presence of chilled margins, the vesiculation of the core of the basic parts, suggest that these different rock types were magmatic at the same time although under marked thermal disequilibrium. Xenocrysts are common: partly resorbed sanidine rimmed with plagioclase in a basic matrix, reactional olivines in tridymite-bearing trachytes. This shows that the mixing occurred between partially crystallized and fractionated magmas. Chemical compositions are continuously variable from basalts to trachytes within the same eruptive cycle. All these facts might be interpreted as the result of a mechanical intermingling, more or less achieved, between two end-members of contrasting composition in the reservoir. In the Sancy Volcano this phenomenon occurred at least, 4 or 5 times during a 600 000 y span. Mixing evidence in Hercynian granodiorites from France. Similar observations may be described in granodioritic rocks. The more widespread “enclaves” exhibit the same characteristics as their volcanic equivalents. Although most of them are small, more important volumes of basic rocks may be associated in the same massif. In the neighbourhood of such large bodies, swarms of smaller enclaves recall the generation of basic pillows. The nature of the contact between small “microgranular enclaves” and their host rocks, the occurrence of xenocrysts with reaction rims (rapakivi feldspars and quartz ocelli in a basic surrounding, amphibole-pyroxene clots in a granitic matrix) and geochemical variations, again suggest thermal and mineralogical disequilibrium. Such observations indicate the comagmatic character of enclaves with regard to their host rocks. They cannot be interpreted as restites and are best described as co-igneous inclusions resulting from non achieved mixing between coexisting magmas. Photonic rocks are less suitable to demonstrate such petrogenetic processes because they result from slow cooling and crystallization during a longer residence time in a magmatic chamber. The successive mixing episodes, giving rise to different generations of co-igneous inclusions are also difficult to recognize. On the other hand, in volcanism this process is quenched and periodically sampled by eruptions. In conclusion the heterogeneity and the variability of many intermediate rocks cannot be explained only in terms of simple partial fusion and fractional crystallization processes in a closed system. Volcanic examples point to a complex evolution of magmatic reservoirs that were continuously fractionated, periodically tapped, periodically refilled and mixed.

Petrology of the San Leone magnetite skarn deposit (S.W. Sardinia)

The San Leone magnetite skarn deposit consists of lens-shaped bodies interlayered in a Silurian quartzo-pelitic formation metamorphosed by Hercynian granodiorite. Iron oxide deposits of this type can result from iron being introduced into a limestone by metasomatism in the vicinity of an intrusive. On the other hand, iron-rich sedimentary carbonates can be transformed into oxides in the vicinity of an intrusive. San Leone appears to be a deposit where both phenomena have operated. A two-stage model is proposed to explain the evolution of the deposit.1. The lens-shaped deposits consisted initially of iron-rich carbonate sediments with quartz-pelitic and/or pyroclastic intercalations. Diffusion across layering, taking place at the time of metamorphism, at about 550 degrees C and low pressure led to the formation of monomineralic (mainly hedenbergite) or oligomineralic layers. This required local mobility of silicon and iron. The state of the iron oxidation appears to have remained unchanged during this metamorphic stage. Resulting minerals include hedenbergite, andradite, grandite, epidote, vesuvianite, calcite, and magnetite.2. The hydrothermal stage, taking place at about 450 degrees C and also at low pressure, consisted of two kinds of phenomena: alteration of hedenbergite and other metamorphic silicates into quartz, magnetite, ferrotremolite, ilvaite, babingtonite, fluorite, calcite, and scheelite and replacement of the remaining calcite by magnetite. Iron was introduced into the lenses by infiltration, presumably along bedding planes. The silicates show an unusually high iron content, especially the amphibole which appears to represent the purest ferrotremolite yet reported from nature.

Deformation and Metamorphism in the Lukmanier Region, Central Switzerland

The Lukmanier region in the central-southern Swiss Alps contains parts of three important structural and stratigraphic units: the Gotthard Massif in the north, the parautochthonous Mesozoic cover of this massif, and the Lucomagno Massif, a frontal part of the Lower Pennine nappe complex to the south. Two successive phases of Alpine folding have been recognized in the Mesozoic cover, both of which occurred after the main northward horizontal transport of the Lower Pennine nappes. The earlier phase, called Phase B, produced three large-scale fold zones in the cover, with a penetrative axial-plane schistosity that strikes east-west and dips steeply north; fold hinge lines are markedly curved in the plane of the schistosity due to differential strain effects. Deformed fossils and ooids indicate the principal axes of Phase B strain and show that the steeply plunging, micaceous mineral lineation, which is found on most schistosity surfaces, is parallel to Phase B maximum strain extension. Regional Phase B folds were modified to various extents by Phase B movements on tectonic slides that lie parallel to the axial-plane schistosity and form the east-west contacts of both massifs with the Mesozoic rocks. The subsequent phase of deformation, called Phase V, produced no large-scale structures, but corrugated bedding and Phase B schistosity and mineral lineation mainly in the southern half of Lukmanier. Fold axial planes strike southeast and dip gently northeast, and fold axes plunge gently southeast. Complex systems of joints and rare, local strike-slip faults developed after Phase V, probably during postorogenic regional uplift. Effects of both phases of Alpine folding are found in the Gotthard and Lucomagno Massifs. Phase B produced the schistosity that strikes east-west and dips steeply north in both massifs, and the shapes of augen, together with deformed schlieren in the Hercynian granodiorite in the Gotthard Massif, indicate that the orientations of Phase B strain axes are similar to those in the Mesozoic cover, with the mineral lineation on the schistosity in both Massifs being parallel to Phase B maximum strain extension. Very intense Phase B deformation in the Gotthard Massif was restricted mainly to mylonitization in narrow east-west zones, and such movements in one zone of ancient paragneisses appear to have controlled the position of the Scopi synclinal zone, the most northerly large-scale Phase B fold zone in the Mesozoic cover. Phase V deformation had little effect in the Gotthard Massif, but Phase V folds are found everywhere in the Lucomagno Massif: their orientation is similar to Phase V folds in the Mesozoic cover. Complex systems of joints in both Massifs probably developed at about the same time as similar joints in the Mesozoic cover. Alpine metamorphism, generally of lower almandine-amphibolite facies, produced a diversity of porphyroblasts in the Mesozoic cover, and they show a variety of textural relations with phases of deformation. These relations change across the area and are due to migration of phases of deformation from south to north relative to stationary metamorphic conditions. Porphyroblast textures also show that shape and predeformational orientation may be important factors controlling amounts of porphyroblast rotation. Regional considerations of Alpine events in Lukmanier and adjacent areas lead to the conclusion that the main northward horizontal transport of the Lower Pennine nappes preceded Phase B deformation in Lukmanier, and in turn Phase B preceded and initiated, by uplift of the Gotthard Massif, the main northward horizontal transport of the Ultrahelvetic and Helvetic nappes.