The in-depth investigation of fractured reservoirs is mainly limited to geophysical data that is in 3D and mostly on the scale of hundred meters to several kilometers or boreholes data that is in 1D and at meter to lower scale. The study of outcropping analogues of buried reservoirs is therefore a key tool for the characterization of the fault and fracture network at the reservoir scale. Tamariu granite has been the subject of this study with the aim to analyse faults and fractures from seismic to borehole scale. With the combination of satellite picture at different resolution and field study, we perform a statistical analysis focused of the length and orientation from infra centimeter crack to hundred kilometer length fault. On the whole range of scale studied, i.e. on 7 orders of magnitude, we have defined a length distribution following a power-law with an exponent a = −2. On the contrary to the length that can be modelled with a unique law, the orientation data shows a variation depending on the scale of observation: as the fault and fracture sets are suitable from the regional faults to the centimeter crack, the proportion of the sets varies at each scale of observation.
Despite quartz being a ubiquitous mineral in the Earth's crust, only little is known about its trace chemical composition and whether and how this is influenced by the medium from which it precipitated. Using in-situLA-ICP-MS analysis, we investigated the trace element composition of several types of quartz from a magmatic-hydrothermal system in the Echassières district of the French Massif Central. This system consists of the Colettes granite, the Beauvoir rare-element granite, a W-bearing stockwork and dispersed mineralized veins. All of the elements tested (Li, Be, B, Na, Al, Ti, Ge, Rb, Sr, Nb, Sn, Sb, Ta, W)were detected in quartz from at least one sample. A rigorous multivariate statistical approach permitted to assign a specific chemical signature to each of the quartz groups that were recognized by petrographic observations. Differences among these groups were interpreted to reflect different sources or genetic evolutionary trends within magmatic and hydrothermally derived quartz groups. For example, the evolution of Al, Rb, Ge, Li and Ti, could be related to genetic processes such as the degree of magma differentiation and crystallization temperatures. Quartz from the W-stockwork, anterior to the Beauvoir leucogranite, clearly stands out from that related to the Beauvoir mineralization system, i.e.,the greisen facies and proximal Ta-Nb-Sn-W mineralized veins. It was also possible to demonstrate a similarity in composition, notably the Sb contents, between quartz from the greisen and from distal stibnite-bearing veins. In addition, elevated values of Sb, Sn as well as Li and Rb in quartz correspond to similarly high values of these elements in the rock, indicating that quartz trace chemistry can provide useful information for the mineralization potential of its host, at least for these elements. On the other hand, Nb, Ta and W are less reliable, as they occur mostly at very low levels in quartz, the highest values corresponding to samples that were not particularly enriched in these metals. A comparison of our quartz data with those from a similar granitic complex, at Cínovec (Czech Republic, ), shows a similar trend for the Ge/Ti ratio, reflecting magmatic differentiation. Conversely, the Al/Ti ratio values differ in the two complexes, the Al content of quartz from the various Cínovec granites being systematically lower than in quartz from Echassières. Nevertheless, caution must be taken when analyzing quartz for its trace element chemistry, as processes occurring during (i.e.,trapping of fluid inclusions and formation of zoning) and after crystal growth (i.e.,diffusion and alteration), can introduce heterogeneities thus affecting the original chemistry.
At low temperatures (<750 degrees C at moderate to high crustal pressures), the production of sufficient melt to reach the melt connectivity transition (similar to 7 vol%), enabling melt drainage, requires an influx of aqueous fluid along structurally controlled pathways or recycling of fluid via migration of melt and exsolution during crystallization. At higher temperatures, melting occurs by fluid-absent reactions, particularly hydrate-breakdown reactions involving micas and/or amphibole in the presence of quartz and feldspar. These reactions produce 20-70 vol%, melt according to protolith composition, at temperatures up to 1000 degrees C. Calculated phase diagrams for pelite are used to illustrate the mineralogical controls on melt production and the consequences of different clockwise pressure-temperature (P-T) paths on melt composition. Preservation of peritectic minerals in residual granulites requires that most of the melt produced was extracted, implying a flux of melt through the suprasolidus crust, although some may be trapped during transport, as recorded by composite migmatite-granite complexes. Peritectic minerals may be entrained during melt drainage, consistent with observations from leucosomes in migmatites, and dissolution of these minerals during ascent may be important in the evolution of some crustal magmas. Since siliceous melt wets grains, suprasolidus crust may become porous at only a few volume % melt, as evidenced by microstructures in residual migmatites in which quartz or feldspar pseudomorphs form after melt films and pockets. With increasing melt volume and decreasing effective pressure, assuming the residue is able to deform and compact, the source becomes permeable at the melt connectivity transition. At this threshold, a change from distributed shear-enhanced compaction to localized dilatant shear failure enables melt segregation. The result is a highly permeable vein network that allows transfer of melt to ascent conduits at the initiation of a melt-extraction event. Melt is drained from the anatectic zone via several extraction events, consistent with evidence for incremental construction of plutons from multiple batches of magma. Buoyancy-driven magma ascent occurs via dikes in fractures or via high-permeability zones controlled by tectonic fabrics; the way in which these features relate to compaction and the generation of porosity waves is discussed. Emplacement of laccoliths (horizontal tabular intrusions) and wedge-shaped plutons occurs around the ductile-to-brittle transition zone, whereas steep tabular sheeted and blobby plutons represent back freezing of melt in the ascent conduit or lateral expansion localized by instabilities in the magma-wall-rock system, respectively.
S-type granites always contain more Al than the amounts of Na, Ca and K in the rock required to form feldspars, primarily owing to their derivation from source components that had previously been weathered. Those rocks are therefore always saturated in Al, or peraluminous. Many I-type granites are also peraluminous, despite I-type source rocks typically not being saturated in Al. It has previously been suggested that this may result from the fractional crystallisation of amphibole. However, data from compositionally zoned high-temperature plutons in the Lachlan Fold Belt show that it is difficult to generate large quantities of peraluminous melt by removal of amphibole. Most of the I-type granites (~ 95%) in the Lachlan Fold Belt formed at lower temperatures and almost half of those rocks for which bulk chemical compositions are available are peraluminous. Among these granites there are 98 separate suites for which there are chemical data for two or more samples, with 47 suites that include both metaluminous more mafic and peraluminous more felsic compositions. The origin of those peraluminous compositions is fundamental to any understanding of I-type granite petrogenesis in this region. Compositional variations caused by the assimilation or by partial melting of supracrustal rocks are very small as the isotopic variations within these rocks are dominantly between different suites, not within suites. The partial melting of more mafic source rocks, rather than the fractional crystallisation of more mafic magmas, is favoured for the origin of these rocks. Partial melting is the most likely process involved in the petrogenesis of felsic granites where broadly granodioritic–monzogranitic batholiths are associated with lesser amounts of tonalite and very minor amounts of mafic rock. Experimental studies have shown that the melts generated by the partial melting of basaltic to andesitic rocks under crustal conditions are mostly peraluminous. During the dehydrational melting of I-type granite source rocks at pressures below the garnet stability field, biotite and amphibole melt incongruently to yield pyroxenes. The excess Al is incorporated into the felsic liquid, resulting in the generation of peraluminous melts. In this instance, the excess Al in felsic I-type granites is a function of the melting process, and unrelated to the bulk composition of the source. The observed gradation from peraluminous felsic granites to metaluminous compositions in less felsic rocks in largely isotopically closed systems could happen in two ways. At higher temperatures of partial melting, Ca and other components of clinopyroxene could dissolve in the melt, with the melt eventually becoming metaluminous. Alternatively, minerals residual from partial melting, dominantly pyroxenes and plagioclase, could be incorporated in suspension in the melt, so that the resulting bulk magma is metaluminous. Examples of these two extremes, and of intermediate cases, are well developed among the granites of southeastern Australia. ► I-type granites of the LFB range from strongly metaluminous to weakly peraluminous. ► Fractional crystallisation of mafic magma is inefficient for forming peraluminous granites. ► Approximately half of I-type granites of the Bega Batholith are peraluminous. ► Peraluminous granites can form by partial melting of metaluminous crustal sources. ► Crustal metaluminous magmas can form by entrainment or dissolution of clinopyroxene.