Amphibole-plagioclase Thermometry Research Articles

Enhancement of Permeability Activated by Supercritical Fluid Flow through Granite

This geological study utilized electron probe microanalysis of granitic rocks to evaluate traces of hydrothermal fluid activity. Amphibole-plagioclase thermometry was applied to estimate the temperature of a glassy vein as approximately 700°C. The results of mesoscopic and microscopic observations of the rock core obtained through borehole investigations revealed that the track of supercritical fluid flow was microfracture filling with hornblende and plagioclase. Grain-boundary microfractures and parallel microfractures were recognized as traces formed by the limited activity of the supercritical fluid immediately after granite setting in the Late Cretaceous. The current high permeability of a borehole in and around the track of supercritical fluid flow was recognized to be related to the microfracture network. In order to investigate the enhancement of permeability activated by the supercritical fluid flow through granite, the results of this geological study and existing data from in situ permeability tests were analysed. Various fractures in and around the trace of a self-sealing zone were investigated for another borehole rock core. The trace of the self-sealing zone, which was composed of filling textures associated with the supercritical fluid, corresponded to the current low-permeability section of the borehole. Representative types were proposed for simple classification based on the characteristics of fractures and the permeability data of each test section. A high-angle fracture of chlorite filling in combination with an open fracture and the development of a sericite-filling fracture network including a low-angle open fracture were recognized as characteristics of high-permeability types. The results of this study indicate that the enhancement of permeability was activated by supercritical fluid flow through granite.

Estimating P-T metamorphic conditions on the roof of a hidden granitic pluton: an example from the Mt. Calamita promontory (Elba Island, Italy)

The metamorphic basement (i.e. Palaeozoic micaschist, Porto Azzurro Unit) cropping out in the Mt. Calamita promontory, in the southeastern Elba Island, represents the roof of the mostly unexposed late Miocene Porto Azzurro granite. A thermobarometrical study was performed on micaschists and interlayered amphibolites to constrain P-T conditions during contact metamorphism. Adopted methods were based on P-T grids, conventional thermobarometry and analysis of fluid inclusions. It was found that the basement was originally located at a depth of 6.5 ± 1.3 km and that the peak metamorphic conditions were close to the muscovite dehydration reaction in micaschist.Therefore, small differences of temperatures conditioned the presence or absence of K-feldspar in the eastern and western side of the Mt. Calamita promontory, respectively. A maximum peak temperature of c. 625 °C was estimated by amphibole-plagioclase thermometry in amphibolite. Fluid inclusions in micaschists quartz lenses document significant episodes of fluid circulation during the cooling history of the metamorphic aureole. Finally, a conductive thermal model was built up to reproduce the thermal evolution during the contact metamorphic event and following cooling. Simulated temperatures resulted significantly lower than the observed peak values. This discrepancy is explained as an effect of the localized heat advection in zones affected by infiltration of magmatic fluids and by the intrusion of numberless felsic dykes.

Petrochemistry and zircon U-Pb geochronology of granitic rocks in the Wang Nam Khiao area, Nakhon Ratchasima, Thailand: Implications for petrogenesis and tectonic setting

Carboniferous biotite granite, Late Permian hornblende granite, and Triassic biotite-hornblende granite, all of which belong to the Eastern Granite Belt, expose in the Wang Nam Khiao area, Nakhon Ratchasima, northeastern Thailand. The Carboniferous biotite granite is dominated by quartz, K-feldspar, plagioclase, and biotite. The Late Permian hornblende granite contains dominant assemblages of plagioclase, quartz, K-feldspar, hornblende, and minor amount of biotite, while the Triassic biotite-hornblende granite consists of quartz, plagioclase, K-feldspar with small amounts of biotite, and hornblende. The REE patterns with steep decrease from light to heavy REE together with the LILE (e.g. K, Sr) enrichment and depletion of some particular HFSE (e.g. Nb, Ti) indicate low degree of partial melting. Mineral chemistry of biotite and hornblende in the granites reflects crystallization from hydrous calc-alkaline arc-derived magmas possibly formed by subduction. Amphibole-plagioclase thermometry and Al-in-hornblende barometry indicate that the Late Permian hornblende granite and the Triassic biotite-hornblende granite may have equilibrated at 3.0–5.8kbar/700–820°C and 2.0–3.2kbar/600–750 °C, respectively, in the middle-upper crust (about 10–15km depth). Zircon U-Pb geochronology of the Carboniferous biotite granite, Late Permian hornblende granite and Triassic biotite-hornblende granite yielded intrusion ages of 314.6–284.9Ma, 253.4Ma, and 237.8Ma, respectively, which implies multiple episodes of arc-magmatism formed by Palaeo-Tethys subduction beneath Indochina Terrane during Late Carboniferous/Early Permian, Late Permian and Middle Triassic.

An experimental study of amphibole stability in low-pressure granitic magmas and a revised Al-in-hornblende geobarometer

We report new experimental data on the composition of magmatic amphiboles synthesised from a variety of granite (sensu lato) bulk compositions at near-solidus temperatures and pressures of 0.8–10 kbar. The total aluminium content (Al^(tot)) of the synthetic calcic amphiboles varies systematically with pressure (P), although the relationship is nonlinear at low pressures (<2.5 kbar). At higher pressures, the relationship resembles that of other experimental studies, which suggests of a general relationship between Al^(tot) and P that is relatively insensitive to bulk composition. We have developed a new Al-in-hornblende geobarometer that is applicable to granitic rocks with the low-variance mineral assemblage: amphibole + plagioclase (An_(15–80)) + biotite + quartz + alkali feldspar + ilmenite/titanite + magnetite + apatite. Amphibole analyses should be taken from the rims of grains, in contact with plagioclase and in apparent textural equilibrium with the rest of the mineral assemblage at temperatures close to the haplogranite solidus (725 ± 75 °C), as determined from amphibole–plagioclase thermometry. Mean amphibole rim compositions that meet these criteria can then be used to calculate P (in kbar) from Al^(tot) (in atoms per formula unit, apfu) according to the expression: P (kbar)=0.5+0.331(8)×Al^(tot) +0.995(4)×(Al^(tot))^2. This expression recovers equilibration pressures of our calibrant dataset, comprising both new and published experimental and natural data, to within ±16 % relative uncertainty. An uncertainty of 10 % relative for a typical Al^(tot) value of 1.5 apfu translates to an uncertainty in pressure estimate of 0.5 kbar, or 15 % relative. Thus the accuracy of the barometer expression is comparable to the precision with which near-solidus amphibole rim composition can be characterised.

Petrology and geochemistry of calc-alkaline volcanic and subvolcanic rocks, Dalli porphyry copper–gold deposit, Markazi Province, Iran

Early Miocene igneous rocks associated with the Dalli porphyry ore body are exposed within the Urumieh-Dokhtar Magmatic Arc (UDMA). The Dalli porphyry Cu–Au deposit is hosted by subduction-related subvolcanic plutons with chemical composition from diorite to granodiorite, which intruded andesitic and dacitic volcanic rocks and a variety of sedimentary sequences. 40Ar/39Ar age data indicate a minimum emplacement age of ∼21 million years for a potasically altered porphyritic diorite that hosts the porphyry system. The deposit has a proven reserve of 8 million tonnes of rock containing 0.75 g/t Au and 0.5% Cu. Chondrite-normalized rare earth element (REE) patterns for the subvolcanic rocks are characterized by light REE enrichments [(La/Sm) n = 2.57–6.40] and flat to gently upward-sloping profiles from middle to heavy REEs [(Dy/Yb) n = 0.99–2.78; (Gd/Yb) n = 1.37–3.54], with no significant Eu anomalies. These characteristics are generated by the fractionation of amphibole and the suppression of plagioclase crystallization from hydrous calc-alkaline magmas. In normalized multi-element diagrams, all analysed rocks are characterized by enrichments in large ion lithophile elements and depletions in high field strength elements, and display typical features of subduction-related calc-alkaline magmas. We used igneous mineral compositions to constrain the conditions of crystallization and emplacement. Biotite compositions plot above the nickel–nickel oxide (NNO) buffer and close to oxygen fugacity values defined by the hematite–magnetite (HM) buffer, indicating oxidizing conditions during crystallization. Assuming a minimum crystallization temperature of 775°C, the oxygen (fO2) and water (fH2O) fugacities are estimated to be 10−10.3 bars (∼ΔNNO+4) and ≤748 bars, respectively, during the crystallization of biotite phenocrysts. The temperature and pressure conditions, estimated from temperature–corrected Al-in-hornblende barometry and amphibole-plagioclase thermometry, suggest that the hornblende phenocrysts in Dalli rocks crystallized at around 780 ± 20°C and 3.8 ± 0.4 kbar. An alternative method using the calcic amphibole thermobarometer indicates that the Dalli magmas were, on average, characterized by an H2O content of 4.3 wt.%, a relatively high oxygen fugacity of 10−11.0 bars (ΔNNO+1.3), and a hornblende phenocryst crystallization temperature of 880 ± 68°C and pressure of 2.6 ± 1.7 kbar.

APG: AN EFFICIENT SOFTWARE PROGRAM FOR AMP-PL THERMOBAROMETRY BASED ON GRAPHICAL METHOD

Geothermobarometry equations are based on thermodynamic principles and appear in single or multi-variant functions. The number of variants for a specific composition or reaction usually is reduced into 2 involving temperature (T) and pressure (P). Since most of planned equations have two passive or variant P and T, using these equations should be with special care. It is very effective to use graphical method to apply geothermobarometry functions. Graphical method is a fundamental way to solve the math functions. In the graphical method two consistent geothermobarometry equations, which both have at least one share variant (P or T), are selected in order to achieve P and T. These selected equations should be applicable for the same reaction or rock. In this method selected thermobarometry equations are drawn on a P-T diagram and then intersection point of them which introduces asked temperature and pressure will be obtained. One of the most common geothermobarometry equations, suitable for intermediate magmatic rocks, is amphibole-plagioclase thermometry and amphibole barometry. APG software program introduced in this paper is specialized and designed for calculating equilibrium temperature and pressure of amphibole and plagioclase within an igneous rock. In this software program, Pressure and temperature are estimated coincidently based on the graphical method.

Geochemical homogeneity of a long-lived, large silicic system; evidence from the Cerro Galán caldera, NW Argentina

By applying a number of analytical techniques across a spectrum of spatial scales (centimeter to micrometer) in juvenile components, we show that the Cerro Galan volcanic system has repeatedly erupted magmas with nearly identical geochemistries over >3.5 Myr. The Cerro Galan system produced nine ignimbrites (∼5.6 to 2 Ma) with a cumulative volume of >1,200 km3 (DRE; dense rock equivalent) of calc-alkaline, high-K rhyodacitic magmas (68–71 wt.% SiO2). The mineralogy is broadly constant throughout the eruptive sequence, comprising plagioclase, quartz, biotite, Fe–Ti oxides, apatite, and titanite. Early ignimbrite magmas also contained amphibole, while the final eruption, the most voluminous Cerro Galan ignimbrite (CGI; 2.08 ± 0.02 Ma) erupted a magma containing rare amphibole, but significant sanidine. Each ignimbrite contains two main juvenile clast types; dominant “white” pumice and ubiquitous but subordinate “grey” pumice. Fe–Ti oxide and amphibole-plagioclase thermometry coupled with amphibole barometry suggest that the grey pumice originated from potentially hotter and deeper magmas (800–840°C, 3–5 kbar) than the more voluminous white pumice (770–810°C, 1.5–2.5 kbar). The grey pumice is interpreted to represent the parental magmas to the Galan system emplaced into the upper crust from a deeper storage zone. Most inter-ignimbrite variations can be accounted for by differences in modal mineralogy and crystal contents that vary from 40 to 55 vol.% on a vesicle-free basis. Geochemical modeling shows that subtle bulk-rock variations in Ta, Y, Nb, Dy, and Yb between the Galan ignimbrites can be reconciled with differences in amounts of crystal fractionation from the “grey” parent magma. The amount of fractionation is inversely correlated with volume; the CGI (∼630 km3) and Real Grande Ignimbrite (∼390 km3) return higher F values (proportion of liquid remaining) than the older Toconquis Group ignimbrites (<50 km3), implying less crystal fractionation took place during the upper-crustal evolution of these larger volume magmas. We attribute this relationship to variations in magma chamber geometry; the younger, largest volume ignimbrites came from flat sill-like magma chambers, reducing the relative proportion of sidewall crystallization and fractionation compared to the older, smaller-volume ignimbrite eruptions. The grey pumice clasts also show evidence of silicic recharge throughout the history of the Cerro Galan system, and recharge days prior to eruption has previously been suggested based on reversely zoned (OH and Cl) apatite phenocrysts. A rare population of plagioclase phenocrysts with thin An-rich rims in juvenile clasts in many ignimbrites supports the importance of recharge in the evolution and potential triggering of eruptions. This study extends the notion that large volumes of nearly identical silicic magmas can be generated repeatedly, producing prolonged geochemical homogeneity from a long-lived magma source in a subduction zone volcanic setting. At Cerro Galan, we propose that there is a zone between mantle magma input and upper crustal chambers, where magmas are geochemically “buffered”, producing the underlying geochemical and isotopic signatures. This produces the same parental magmas that are delivered repeatedly to the upper crust. A lower-crustal MASH (melting, assimilation, storage, and homogenization) zone is proposed to act as this buffer zone. Subsequent upper crustal magmatic processes serve only to slightly modify the geochemistry of the magmas.

P– T conditions of crystallization and origin of plagioclase-mantled alkali feldspar megacrysts in the Mesozoic granitoids in the Qinling orogen (China)

The Qinling orogen between the North China and South China cratons was intruded at 211–217 Ma by calc-alkaline quartz monzonitic to monzogranitic plutons characterized by I- to A-type geochemistry and in many places contain plagioclase-mantled alkali feldspar megacrysts (rapakivi texture sensu lato). The felsic rocks contain mafic to intermediate magmatic enclaves suggestive of mingling and mixing of mafic and felsic magmas. The P– T conditions of crystallization have been determined for early mineral assemblages (inner parts of alkali feldspar megacrysts and their plagioclase, quartz, amphibole and biotite inclusions) and late assemblages (matrix minerals) of the rapakivi-textured granitoids. Al contents in amphibole from the early and late mineral assemblages yield pressures of 1.2–3.0 and 0.7–3.0 kbar, respectively, and indicate only minor pressure change between the crystallization of the early and late assemblages. Amphibole–plagioclase thermometry gives temperatures mainly of the order of 900 to 1000 °C for both the early and late assemblages indicating nearly isothermal conditions. Feldspar thermometers yield lower temperatures. Relative abundances of minerals and their chemical compositions indicate that the late mineral assemblages tend to be richer in MgO, Na 2O and CaO than the early assemblages. Rapakivi texture is interpreted in this case mainly as a result of compositional changes related to the hybridization between granitic and more mafic magmas. Small release of pressure during crystallization of the magmas may have contributed to the origin of the mantled alkali feldspar megacrysts.

Mineral chemistry of the Qitianling granitoid and the Furong tin ore deposit in Hunan Province, South China: implication for the genesis of granite and related tin mineralization

The Qitianling granitic stock is a stanniferous granite located in Hunan Province, China. Recently, the Furong tin ore deposit was found in this stock, with a tin reserve of approximately 600,000 tons. The main rock-forming minerals including amphibole, biotite, plagioclase and ore minerals including cassiterite and rutile were analysed chemical compositions by electron microprobe. The biotite is Fe-rich annite, and has high Ti and Cl concentration. The biotite has high Fe 3+ /(Fe 2+ +Fe 3+ ) ratios and the oxygen fugacity calculated by biotite compositions is above Ni-NiO (NNO), and near the Fe 2 O 3- Fe 3 O 4 (MH). The amphiboles are ferropargasite and ferro-edenite hornblende. The pressure of the granite estimated by Al-in-hornblende barometer is 3.6 ± 0.9 kbar. An amphibole-plagioclase thermometry and a semiquantitative hornblende thermometer yield a forming temperature of 750∼820°C. The chlorite from the orebody has negligible K 2 O, Ti 2 O, F, and Cl, but shows similar Fe/(Fe+Mg) ratios with amphibole and biotite. Cassiterite is observed closely associated with rutile disseminated in chlorite alteration veins and envelopes. The ore-forming temperature is estimated to be 290∼405°C from chlorite geothermometry. The Qitianling granite contains Sn-rich hornblende and biotite, and shows a high oxygen fugacity for the magma, which is slightly different from the common S-type stanniferous granites worldwide. Fractional crystallization of the magma and tin deposition directly from exsolved magmatic-hydrothermal fluids may not be the major mechanism for the tin mineralization in this deposit. Instead, we suggest that post-magmatic hydrothermal alteration of the granite may have released tin and other metals ( e.g. , Ti) from the Sn-bearing biotite and hornblende in the granite. Then cassiterite and rutile precipitated together with chlorite when the physical and chemical condition of the Sn- and Ti-rich fluids changed.

Near-solidus evolution of oceanic gabbros: insights from amphibole geochemistry

The near-solidus evolution of plutonic rocks formed at slow-spreading ridges is investigated using the major and trace element compositions of amphiboles in a suite of gabbros from the Mid-Atlantic Ridge. These new data allow unambiguous geochemical discrimination between amphiboles of magmatic and hydrothermal origin. In turn, this allows the gabbro solidus to be constrained to 860 ± 30°C, using amphibole-plagioclase thermometry. This is consistent with temperatures from associated secondary clinopyroxene. Magmatic amphibole, which can be identified in almost all samples, formed during metasomatism of a low-porosity crystal mush by an evolved hydrous silicate melt. These amphiboles are characterised by high F, Nb, and F/Cl and low Cl contents. The amphibole-forming reaction involved melt, plagioclase, and clinopyroxene. Amphibole blebs with a geochemically magmatic signature are found enclosed in the cores of some primitive clinopyroxene crystals. There is no evidence for a seawater component in the magmatic amphibole, as would be expected if high-temperature seawater ingress leads to flux melting, as has recently been suggested. However, the ingress of seawater-derived fluids did occur at temperatures within error of the gabbro solidus forming amphibole in veins and replacing igneous phases. These amphiboles are characterised by high Cl, B, and Cl/F and low Nb, F, and Nb/La. The fluids involved in the formation of these amphiboles had compositions unlike seawater or hydrothermal vent fluids.

Crystallization conditions of the Wiborg rapakivi batholith, SE Finland: an evaluation of amphibole and biotite mineral chemistry

The wiborgite and dark wiborgite rapakivi granite phases of the Wiborg batholith in southeastern Finland compose about 80% of the total batholith area. A new study of the dominant mafic silicate minerals, in comparison with mafic silicates from more evolved granite phases, hybridized granite and mafic magmatic enclaves provide insights into the overall petrogenesis of the Wiborg batholith. All of the mafic silicate minerals are iron-rich, reflective of the whole rock compositions. Biotite is annitic, calcic amphibole is ferro-edenite to hastingsite, and subsolidus Fe-Mg amphibole is found as accessory grunerite. Temperatures derived from amphibole-plagioclase thermometry suggest crystallization at about ∼ 740 °C. Pressure estimates derived from Al in amphibole barometry range between 2.5 and 5.4 kilobars. This is noticeably higher than the previous estimates of 1 kbar for the Wiborg batholith. Oxygen fugacity estimates from biotite suggest low fO2 initial values and increase from FMQ to above NNO for late stage granite phases.

Contrasting pressure–temperature–deformation history across a vestigial craton–mobile belt boundary: the western margin of the Eastern Ghats Belt at Deobhog, India

At Deobhog, migmatitic gneisses and granulites of the Eastern Ghats Belt are juxtaposed against a cratonic ensemble of banded augen gneiss, amphibolite and calcsilicate gneiss, intruded by late hornblende granite and dolerite. In the migmatitic gneiss unit, early isoclinal folds (syn‐D1M and D2M) are reoriented along N–S‐trending and E‐dipping shear planes (S3M), with (S1M–S3M) intersection lineations having steep to moderate plunges. The near‐peak P–T condition was syn‐D3M (≥900 °C, 9.5 kbar), as inferred from syn‐D3M Grt+Opx‐bearing leucosomes in mafic granulites, and from thermobarometry on Grt (corona)–Opx/Cpx–Pl–Qtz assemblages. The P–T values are consistent with the occurrence of Opx–Spr–Crd assemblages in spatially associated high‐Mg–Al pelites. A subsequent period of cooling followed by isothermal decompression (800–850 °C, c. 7 kbar) is documented by the formation of coronal garnet and its decomposition to Opx+Pl symplectites in mafic granulites. Hydrous fluid infiltration accompanying the retrograde changes is manifested in biotite replacing Opx in some lithologies.The cratonic banded gneiss–granite unit also documents two phases of isoclinal folding (D1B &amp; D2B), with the L2B lineation girdle different from the lineation spread in the migmatitic gneiss unit. Calcsilicate gneiss (Hbl–Pl–Cpx–Scap–Cal) and amphibolite (Hbl–Pl±Grt±Cpx) within banded gneisses record syn‐D2B peak metamorphic conditions (c. 700 °C, 6.5 kbar), followed by cooling (to c. 500 °C) manifested in the stabilization of coronal clinozoisite–epidote. The D3B shear deformation post‐dates granite and dolerite intrusions and is characterized by top‐to‐the‐west movement along N–S‐trending, E‐dipping shear planes. Deformation mechanisms of quartz and feldspar in granites and banded gneisses and amphibole–plagioclase thermometry within shear bands in dolerites document an inverted syn‐D3B thermal gradient with temperature increasing from 350 to 550 °C in the west to ≥700 °C near the contact with the migmatitic gneiss unit. The thermal gradient is reflected in the stabilization of chlorite after hornblende in S3B shears to the west, and post‐D2B neosome segregation along D3B folds and shears to the east.The contrasting lithologies, early structures and peak metamorphic conditions in the two units indicate unconnected pre‐D3P–T –deformation histories. The shared D3 deformation in the two units, the syn‐D3 inverted thermal gradient preserved in the footwall cratonic rocks and the complementary cooling and hydration of the hanging wall granulites across the contact are attributed to westward thrusting of ‘hot’ Eastern Ghats granulites on ‘cool’ cratonic crust. It is suggested that the Eastern Ghats migmatitic gneiss unit is not a reworked part of the craton, but a para‐autochthonous/allochthonous unit emplaced on and amalgamated to the craton.

Regional tilt of the Mount Stuart batholith, Washington, determined using aluminum-in-hornblende barometry: Implications for northward translation of Baja British Columbia

We have developed a new quantitative method to estimate paleohorizontal in granitic plutons using the aluminum-in-hornblende (AH) barometer. The method is used to correct previously published paleomagnetic data from the 93‐96 Ma Mount Stuart batholith of the Cascades Mountains, Washington State, for the effects of postemplacement tilting. AH barometry was done on 46 samples from the batholith using the compositions of hornblende rims coexisting with the full mineral assemblage required for pressureestimation.High-contrastback-scatteredelectronimagingwas usedtoensurethattheanalyzedhornblendeswerenotsignificantly affected by subsolidus alteration. The success of the AH barometry is indicated by two observations. First, increases in the Al content of the hornblendes are governed almost entirely by a pressure-sensitive tschermak-type substitution. Second, amphibole-plagioclase thermometry indicates that assemblage equilibration occurred at or very near magmatic conditions (’650 &C) and that temperature has a negligible effect on our pressure estimates. AH barometry results indicate that the depth of crystallization across the batholith decreases systematically from ’0.3 GPa in the northwest to ’0.15 GPa in the southeast,consistentwithindependentbarometryforthecontactaureole ofthebatholithandregionalstructuralandstratigraphicrelations. Using a best-fit planar-tilt model and bootstrap analysis of uncertainties, we estimate that the paleohorizontal plane has a strike of 43& 630.4&and dip of 7& 62.0&southeast (695% confidence). Our estimated paleohorizontal allows us to restore the paleomagnetic data of Beck et al. (1981) and to estimate the original paleolatitude of the Mount Stuart batholith. Beck et al. found that the southern part of the batholith yielded a number of sites with a well-defined high-coercivity remanence. The carrier of this remanence was not resolved, but the following four lines of evidence stronglysuggestthatthepublisheddirectionswereacquiredshortly after emplacement of the batholith. (1) The ‘‘stable’’ sites all came from the shallowest and most rapidly cooled portions of the batholith as indicated by our AH results and concordant K/Ar ages for hornblende/biotite pairs. (2) The high coercivity component was always normal in polarity, which is consistent with emplacement of the Mount Stuart batholith at the end of the Cretaceous long normal. (3) Sites from the batholith and the contact aureole gave similar directions. (4) The directions show no indication of tilt-related smearing. After restoration, the paleomagnetic data indicate 42& 6 11& clockwise rotation and 3100 6 600 km of northward offset

Non-ideal interactions in calcic amphiboles and their bearing on amphibole-plagioclase thermometry

Amphibole thermodynamics are approximated with the symmetric formalism (regular solution model for within-site non-ideality and a reciprocal solution model for cross-site-terms) in order to formulate improved thermometers for amphibole-plagioclase assemblages. This approximation provides a convenient framework with which to account for composition-dependence of the ideal (mixing-on-sites) equilibrium constants for the equilibria: For A and B all possible within-site and cross-site interactions among the species □−K−Na−Ca−Mg−Fe2+−Fe3+−Al−Si on the A, M4, M1, M3, M2 and T1 amphibole crystallographic sites were examined. Of the 36 possible interaction energy terms, application of the symmetric formalism results in a dramatic simplification to eight independent parameters. Plagioclase nonideality is modelled using Darken's quadratic formalism. We have supplemented an experimental data set of 92 amphibole-plagioclase pairs with 215 natural pairs from igneous and metamorphic rocks in which the pressure and temperature of equilibration are well constrained. Regression of the combined dataset yields values for the eight interaction parameters as well as for apparent enthalpy, entropy and volume changes for each reaction. These parameters are used to formulate two new thermometers, which perform well (±40°C) in the range 400–1000°C and 1–15 kbar over a broad range of bulk compositions, including tschermakitic amphiboles from garnet amphibolites which caused problems for the simple thermometer of Blundy and Holland (1990). For silica-saturated rocks both thermometers may be applied: in silica-undersaturated rocks or magmas thermometer B alone can be applied. An improved procedure for estimation of ferric iron in calcic amphiboles is presented in the appendix.

Emplacement-related deformation of granitoid magmas, southern Adamello Massif, Italy

The Lago della Vacca Suite in the Tertiary Adamello Massif, Italian Alps, is a small (∼10 km 2 ), crudely zoned granitoid pluton emplaced at a pressure of ∼3.5 kbar into folded Mesozoic sediments and precursor intrusive rocks. Field relations and detailed mapping reveal a temporal evolution in the style of emplacement from early, passive stoping by the mafic to intermediate composition marginal units, to later forceful emplacement of the more silicic core. Marginal units of the pluton have highly discordant, stoped outer contacts against folded Triassic carbonates and precursor mafic/ultramafic rocks. In contrast, the core units have steeply inward-dipping contacts against these marginal units. Magmatic foliations defined by the planar alignment of plagioclase, amphibole, and biotite, and by mafic enclaves, are well developed in all units. They describe steeply inward-dipping concentric trajectories that approximately parallel unit contacts and in general increase in intensity toward the pluton margin. Lineations are rarely present. The most intensely foliated rocks occur in the marginal units, as anastomosing, sinuous bands that wrap around lenses of less intensely foliated granitoid. Three-dimensional strain analysis using mafic enclaves from throughout the pluton indicates that these bands represent zones of very high strain. Strain ellipsoids are weakly prolate within the core of the pluton, becoming increasingly oblate as strain increases toward the pluton margin. The short axis (Z) of the strain ellipsoid is uniformly subnormal to the foliation, suggestive of radial flattening. Textures of the marginal rocks, which record high strain, coupled with amphibole-plagioclase thermometry, indicate that deformation occurred in the magmatic or submagmatic state (∼650-780 °C) and was accompanied by significant migration of interstitial melt. The documented transition from passive to forceful emplacement, and final geometry of the suite, resulted from temporal changes in the rheology of both the magma and adjacent country rocks. Pulses of low-density hydrous magma were initially emplaced by stoping denser country rocks. On cooling and solidification, these magmas contracted by 15%-20%, creating an unpressured volume, which allowed subsequent magma pulses to be emplaced into the pluton core. These pulses distended and flattened their partially molten carapace and those parts of the aureole that had been thermally softened by intrusion of earlier magmas. Emplacement of the Lago della Vacca Suite was therefore the product of several internal or magma-related processes, an interaction between stoping, radial expansion, and volume contraction on solidification of the hydrous magmas. The heterogeneous distribution of strain in the marginal units reflects local variations in temperature and, most importantly, in the distribution of interstitial melt during emplacement.

The metamorphism and migmatization of the Xanthe-Echinos metamorphic complex, Central Rhodope, Greece

The Xanthe-Echinos metamorphic complex in the central Hellenic Rhodope records early Tertiary convergent tectonics on the Eurasian continental margin of Neotethys. It consists principally of layered gneiss and migmatite with minor amphibolite and calc-silicate rocks. Three metamorphic events are recognised: a high-pressure eclogite event (?Cretaceous); medium-pressure, Barrovian-type metamorphism (Eocene); and later low-pressure, greenschist facies metamorphism. Petrographic composition and electron microprobe analysis of minerals in forty four samples of gneiss and migmatite shows that the Eocene Barrovian event is characterized by replacement of clinopyroxene by pargasitic hornblende in clinopyroxene-garnet and clinopyroxene-edenitic hornblende assemblages. The compositional variation and textures of the amphiboles suggest initial growth of edenitic hornblende zoned to hornblende and development of syn- to late-tectonic pargasitic hornblende under higher temperature conditions associated with migmatization. The textures, zoning and paragenetic relationships of sodic-calcic amphiboles are consistent with uplift accompanied by synmetamorphic deformation. Partial melting in the migmatites involved plagioclase+K-feldspar+quartz. The solidification products of the produced melt are present in the form of quartz monzonitic leucosome layers and is also represented in syntectonic plutons. Metamorphic pressures of 7.5–8.5 kbar (at T = 600° C) were estimated from garnet-clinopyroxene pairs in gneisses adjacent to the migmatites. Amphibole-plagioclase thermometry of migmatite samples yielded temperatures of 728°–746°C (at P = 8.5 kbar). The activity of H 2O was probably less than 1.0 in the partially melted rocks. The geothermobarometric results indicate an increase of the geothermal gradient during uplift, probably related to intrusion of basic magma into the crust and/or changes in geothermal gradient associated with crustal thickening.