Clinopyroxene-bearing Rocks Research Articles

Zircon Reveals Diverse Trends of Magma Crystallization from Two Types of Early Post-Collisional Diorites (Variscan Orogen, NE Bohemian Massif)

Abstract Amphibole- and clinopyroxene-bearing monzodiorites were emplaced at 340 Ma (CA-ID-TIMS zircon age), suggesting the formation of hydrous and dry magmas closely related in space and time in the NE Bohemian Massif. Hafnium and oxygen isotopes of zircon in less evolved rocks (<55 wt% SiO2) are similar between Amp and Cpx monzodiorites (εHf = −3.3 ± 0.5 and − 3.5 ± 0.8; δ18O = 6.4 ± 1.0 and 6.8 ± 0.7, respectively), consistent with a common source—a contaminated mafic magma derived from an enriched mantle. At the same time, the conditions of crystallization are distinct and zircon appears to be an excellent tool for distinguishing between hydrous and anhydrous crystallization conditions, a process that may be more ambiguously recorded by whole rock and major mineral chemistry. In particular, elements fractionated by either amphibole or plagioclase crystallization, such as Hf, Dy, and Eu, differ in zircon from amphibole- and clinopyroxene-bearing rocks, and Zr/Hf, Yb/Dy, and Eu/Dy are therefore useful indices of crystallization conditions. We show that the composition of zircon from hydrous dioritic magmas is not comparable with that of typical zircon from dioritic-granitic suites worldwide, suggesting a specific process involved in their formation. Here, we propose that fluid-present remelting of a mafic underplate is necessary to explain the rock textures as well as the composition of the whole rock, zircon, and other minerals of amphibole-bearing monzodiorites and that a similar process may control the formation of amphibole-rich dioritic rocks worldwide, including appinitic suites. Overall, we show that dioritic rocks represent snapshots of differentiation processes that occur in the early stages of magma evolution before the magma is homogenized into large-scale batholiths.

Sulfur and REE zoning in apatite: the example of the Colli Albani magmatic system

We investigate the distribution of major and trace elements in apatite crystals hosted in granular alkaline rocks composed mainly of leucite and clinopyroxene, representative of the hypabyssal crystallization of a magma body in the Quaternary ultra-potassic Colli Albani Volcanic District (CAVD), which was emplaced into thick limestone units along the Tyrrhenian margin of Italy. Results show that the analyzed crystals are the SrO-richest (up to 4.6 wt%) fluorapatite (F =2.6-3.7 wt%) of the Italian alkaline rocks. The strontium enrichment is caused by the lack of other Sr-compatible mineral phases, such as plagioclase, alkali feldspar and melilite, in these leucite- and clinopyroxene-bearing rocks. The studied samples show core-rim zoning with rims enriched in Si, S, and REE whereas the cores are enriched in Ca and P. The LREE-oxides contents of apatite, reaching 4.2 wt%, represent more than 95% of the total REE budget; SiO2 contents range from 1.3 to 3.6 wt%, and SO3 concentrations between 0.6 and 1.4 wt%.

A REE-in-plagioclase–clinopyroxene thermometer for crustal rocks

A REE-in-plagioclase-clinopyroxene thermometer has been developed on the basis of the temperature- and composition-dependent rare-earth element (REE) partitioning between coexisting plagioclase and clinopyroxene. This two-mineral exchange thermobarometer is constructed using parameters from lattice strain models for REE + Y partitioning in plagioclase and in clinopyroxene that were independently calibrated against experimentally determined mineral-melt partitioning data. An important advantage of this REE-based thermometer is that it can provide accurate temperatures through linear least-squares analysis of REE + Y as a group. Applications of the REE-in-plagioclase-clinopyroxene thermometer to volcanic and cumulate rocks show that temperatures derived from the new thermometer agree well with independently constrained magma crystallization temperatures, which adds confidence to applications of the REE-exchange thermometer to natural rocks with a wide spectrum of composition (i.e., from basalt to rhyolite). However, systematic temperature differences appear between the REE- and Mg-exchange thermometers for the volcanic and cumulate rocks. Through numerical simulations of diffusion in plagioclase-clinopyroxene systems, we demonstrate that (1) due to their slower diffusion rates, REE in minerals preferentially records crystallization or near-crystallization temperatures of the rock, and that (2) Mg is readily rest to lower temperatures for rocks from intermediately or slowly cooled magma bodies but records the initial crystallization temperatures of rocks from rapidly cooled magmas. Given their distinct diffusive responses to temperature changes, REE and Mg closure temperatures recorded by the two thermometers can be used in concert to study thermal and magmatic histories of plagioclase- and clinopyroxene-bearing rocks.

Origin of primitive ultra-calcic arc melts at crustal conditions — Experimental evidence on the La Sommata basalt, Vulcano, Aeolian Islands

To interpret primitive magma compositions in the Aeolian arc and contribute to a better experimental characterization of ultra-calcic arc melts, equilibrium phase relations have been determined experimentally for the La Sommata basalt (Som-1, Vulcano, Aeolian arc). Som-1 (Na2O+K2O=4.46wt.%, CaO=12.97wt.%, MgO=8.78wt.%, CaO/Al2O3=1.03) is a reference primitive ne-normative arc basalt with a strong ultra-calcic affinity. The experiments have been performed between 44 and 154MPa, 1050 and 1150°C and from NNO+0.2 to NNO+1.9. Fluid-present conditions were imposed with H2O–CO2 mixtures yielding melt H2O concentrations from 0.7 to 3.5wt.%. Phases encountered include clinopyroxene, olivine, plagioclase and Fe-oxide. Clinopyroxene is slightly earlier than olivine in the crystallization sequence. It is the liquidus phase at 150MPa, being joined by olivine on the liquidus between 44 and 88MPa. Plagioclase is the third phase to appear in the crystallization sequence and orthopyroxene was not found. Experimental clinopyroxenes (Fs7–16) and olivines (Fo78–92) partially reproduce the natural phenocryst compositions (respectively Fs5–7 and Fo87–91). Upon progressive crystallization, experimental liquids shift towards higher SiO2 (up to ~55wt.%), Al2O3 (up to ~18wt.%) and K2O (up to ~5.5wt.%) and lower CaO, MgO and CaO/Al2O3. Experimental glasses and natural whole-rock compositions overlap, indicating that progressive crystallization of Som-1 type melts can generate differentiated compositions such as those encountered at Vulcano. The low pressure cotectic experimental glasses reproduce glass inclusions in La Sommata clinopyroxene but contrast with glass inclusions in olivine which preserve basaltic melts more primitive than Som-1. Phase relations for the La Sommata basalt are identical in all critical aspects to those obtained previously on a synthetic ultra-calcic arc composition. In particular, clinopyroxene+olivine co-saturation occurs at very low pressures (≤100MPa). Ultra-calcic arc compositions do not represent primary mantle melts but result from the interaction between a primary mantle melt and clinopyroxene-bearing rocks in the arc crust. At Vulcano, primitive ultra-calcic end-member melts were generated between 250 and 350MPa in the lower magma accumulation zone by reaction between hot primitive melts and wehrlitic or gabbroic lithologies. At Stromboli, golden pumices and glass inclusions with an ultra-calcic affinity were also generated at shallow pressures, between 150 and 250MPa, suggesting that the interaction model is of general significance in the Aeolian arc.

Structure and evolution of the lithospheric mantle beneath Siberian craton, thermobarometric study

Monomineral thermobarometry (MTB) data derived from EMP analyses for heavy monomineral separates from > 20 Yakutian kimberlite pipes were used to compile a SSE–NNW traverse of the mantle beneath the Siberian craton. Orthopyroxene (Opx) MBT for the mantle section beneath Udachnaya gives three PT paths: a low temperature (LT) conductive branch (Boyd et al., 1997), estimated with thermometers of Krogh (1988) or/and O'Neill and Wood (1979), and two other HT paths. The three paths correspond to different values of Fe# in Ol (0.075, 0.085 and 0.11). They are reproduced by the modified MTB equations for clinopyroxene, garnet, chromite and ilmenites (Ashchepkov et al., 2008a), a mono-version of the O'Neill and Wood (1979) thermometer with corrections to Cr and Ca/Mg ratios which mark conductive geotherm. PT estimates for garnets and pyroxenes reflect mantle layering, whereas those for ilmenite reflect varying conditions of polybaric mantle protokimberlite systems and metasomatism. MTB for xenoliths from the Udachnaya, Mir, Dalnyaya, and Komsomolskaya kimberlites show the colder branches if PT path using heavy mineral analyses them according to PT for xenoliths according to TB (O'Neill and Wood, 1979; Finnerty and Boyd, 1987; McGregor, 1974; Brey and Kohler, 1990) producing smoother geotherms. MTB gives a wider range PT points reflecting heating near magmatic channels. Regularities of mantle sections and layering in the Daldyn field are recognized on the PT and P– X diagrams. The lower part of the mantle sections were heated by protokimberlite melts which created the megacrystalline associations. PT values of sub-calcic garnets correlate with those of picroilmenites. Mantle columns beneath the large pipes reveal stepped 7–12 layering for Udachnaya pipe units correlating with the peaks of Re/Os ages (Griffin et al., 2002b) is marked by periodic increase in Fe# in minerals. Pyroxenites and ‘hot’ eclogites (Pokhilenko et al., 1999) are found in layers at ∼ ∼ 40, 50–65, and ∼ 70 kbar. Sub-continental lithospheric mantle (SCLM) beneath the Alakite field has been subjected to pervasive multistage metasomatism, as indicated by Fe-enriched Cr-diopsides and Ti-rich low-Ca garnets. Ilmenite PT trends were formed by rising protokimberlites that underwent AFC. In the Upper Muna field the mantle is similar in structure to that of the Alakite region. Fe-rich clinopyroxene-bearing rocks (60–55 kbar) are located between the ilmenite-forming systems (70–60 and 55–40 kbar), sub-Ca garnets start from 40 kbar and become more abundant downward. Beneath the Nakyn field, rhythmic layering is found for peridotites in the lower part ( P > 40 kbar), fertilization by Fe–Cpx (40–50 kbar) follow the Ilm-forming system ∼ 55–60 kbar correlating with the occurrence of depleted (low-Ca) peridotites. Beneath the Anabar fields highly depleted mantle at depth > 40 kbar has been subjected to Fe-metasomatism and pervasive metasomatism that accompanied protokimberlite feeders marked by low Cr-ilmenites accompanied by fertilization. In the upper section abundant garnet- and clinopyroxene-rich peridotites are typical. Comparison of mantle sections reconstructed from monomineral PT estimates from Paleozoic and Mesozoic kimberlites show differences in entrainment levels which were elevated after the Permian–Triassic superplume to > 55–40 kbar without delamination.

The petrology of two distinct granulite types in the Hengshan Mts, China, and tectonic implications

The Archean to Proterozoic Hengshan Complex (North China Craton), comprises tonalitic and granodioritic gneisses with subordinate mafic lenses, pegmatites and granites. Amphibolite facies assemblages predominate, although granulite-facies relics are widespread, and greenschist-facies retrogression occurs in km-wide shear zones. Mafic lenses, locally abundant, occur as strongly deformed amphibolite (hornblende+plagioclase) boudins or sheets. In contrast to previously published models we find two series of mafic rocks with distinctly different granulite-facies evolutions. In the north of the Complex, relict high-pressure mafic granulites are garnet+clinopyroxene-bearing rocks with a secondary development of orthopyroxene around both garnet (kelyphites) and clinopyroxene (coronas). South of the newly defined central, E–W-trending, Zhujiafang shear zone, numerous mafic boudins and less-deformed dykes exhibit a macroscopically visible magmatic texture with coronitic growth of metamorphic garnet (full of quartz inclusions) between the magmatic plagioclase and pyroxene domains. Additional orthopyroxene (after magmatic augite) and sodic rims to magmatic plagioclase clearly indicate medium-pressure granulite-facies metamorphism. These findings suggest tectonic juxtaposition in this area of three different structural levels of the same Proterozoic-imprinted crust: high-pressure granulite grade in the northern Hengshan, medium-pressure granulite grade in the southern Hengshan and amphibolite- to greenschist-facies grade in the Wutaishan to the SE.

Eclogites with a short-lived granulite facies overprint in the Moldanubian Zone, Czech Republic: petrology, geochemistry and diffusion modelling of garnet zoning

The petrology and geochemistry of a newly discovered suite of high-pressure garnet + clinopyroxene-bearing rocks from the Monotonous Series of the Moldanubian Zone of the Bohemian Massif, southwest Czech Republic have been investigated. Three types [common eclogites, quartz ± kyanite ± (clino)zoisite eclogites and garnet-hornblende-clinopyroxenites] are distinguished by petrography and geochemistry. All underwent a significant degree of partial breakdown under granulite and amphibolite facies conditions during exhumation. Important features include the growth of orthopyroxene in breakdown domains after garnet and omphacite and anorthite + spinel ± corundum ± exceedingly peraluminous sapphirine replacing kyanite. Garnet zoning and inclusion patterns support a prograde evolution from low pressures for at least some of the samples. The post-eclogite stage granulite facies overprint indicates that high temperatures prevailed during exhumation, but preservation of zoning in some garnets and the results of diffusion modelling suggest that this overprint took place over a very short time-scale. The geochemical and petrological results allow characteristic differences to be recognized between these eclogites and metabasites found in other tectonic units of South Bohemia and consequently the assigning of all high-pressure rocks to a single, now disrupted, tectonic unit is a gross simplification that seriously misrepresents the tectono-metamorphic history of the region.

Compositions and phase relations of calcic amphiboles in epidote- and clinopyroxene-bearing rocks of the amphibolite and lower granulite facies, central Massachusetts, USA

Calcic amphiboles coexisting with epidotegroup minerals (zoisite, clinozoisite, epidote) and/or clinopyroxene±plagioclase±quartz±garnet occur in amphibolites and calc-silicate rocks that underwent amphibolite to lower granulite-facies metamorphism in the Acadian metamorphic high of central Massachusetts, USA. Across the region, peak metamorphic conditions range from about 580° C and 6.2 kbar to 730° C and 6.3 kbar. The coexistence of most Ca-amphiboles with Fe3+-rich epidote-group minerals suggests the presence of Fe3+ in most of these amphiboles. An empirical Fe3+ estimation for the microprobe analyses is based on two constraints: the Na−Ca content of the M4 sites of Ca-saturated, gravimetrically analyzed hornblendes gives the relation: Ca(M4) c =-1.479 Na(M4) c +2 (c=corrected). The second constraint is the stoichiometric equation Ca(M4)+Na(M4)+FM=15, where FM is the sum of all cations exclusive of Ca, Na, and K. Solving the two equations simultaneously gives: 20.185=0.479 Ca(M4)+1.479 ΣFM. Starting with the uncorrected values of Ca(M4) u and ΣFM(M4) u (u = uncorrected) of the all ferrous formula, the normalization factor NF for calculating the corrected cations of the ferric formulas is: 20.185/(0.478 Ca(M4) u +1.479 ΣFM u ). From the deficient oxygen the Fe3+ content which is equal to 2(23-ΣOX) can be calculated. Determinations of Fe3+ contents of four hornblende separates by Mossbauer spectroscopy are in agreement with the calculated values. The Ca-amphiboles show systematic changes in composition with increasing grade of metamorphism within the amphibolite and lower granulite-facies zones: increasing edenite and tschermakite substitution, increasing Ti content, and increasing Fe2+/(Fe2++Mg) ratio. In addition, the coexisting clinopyroxenes are also characterized by an increase in Fe2+/(Fe2++Mg) ratio. In quartz-free rocks with coexisting Ca-amphibole and plagioclase there is an increase in the ratio X Ab/X Ed, where X Ab=Na/(Na+Ca) in plagioclase and X Ed=Na in the amphibole A-site. These chemical changes in mineral composition together with the disappearance of epidote at the transition to granulite-facies metamorphic conditions are attributed to the continuous reaction: albite+epidote+Fe-Mg hornblende→Fe−Mg clinopyroxene+anorthite+(NaAlSi-1)Hbl+H2O.