Rosangela Bocchio

Trace element distribution in eclogites and their clinopyroxene/garnet pair a case study from Soazza (Switzerland)

Rare earth element (REE) abundances have been determined in six samples of metabasites from the Pennidic Adula nappe in the area of Soazza (Mesolcina Valley, Switzerland). The samples cover a broad range of mineralogical composition from eclogite to epidote amphibolite through symplectitic garnet amphibolite. The REE patterns do not vary significantly and indicate a common origin of these rocks as “normal” mid-ocean ridge basalts (N-MORB). Two main eclogitic phases, clinopyroxene (cpx) and garnet (grt), have also been analysed for REE and other trace elements (Sc, V, Cr, Co, Ni, Cu, Zn, Sr, Y, Zr). The incorporation of REE and most trace elements in structural sites of cpx and grt is in accordance with that expected mainly from an evaluation of site geometry, ionic radius and charge balance and is therefore dependent on the major-element composition of the host phases. The changes in cpx/grt partition coefficients (D i ) for HREE are particularly associated with the temperature condition of the equilibration of the pair.

Demantoid from Val Malenco, Italy: Review and Update

GEMS & GEMOLOGY WINTER 2009 emantoid is the Cr-bearing yellowish green to green variety of andradite [Ca3Fe2(SiO4)3] (O’Donoghue, 2006). Very popular in Russia (where it was first discovered) from about 1875 to the start of the Russian Revolution in 1917, this gem has enjoyed a resurgence in demand since the beginning of the 21st century (Furuya, 2007). One of the most notable localities for demantoid is Val Malenco, located in Sondrio Province in the Lombardy region of northern Italy. Several deposits in this area have produced well-formed rhombic dodecahedral crystals (e.g., figure 1, left) that are coveted by collectors (Bedogne and Pagano, 1972; Amthauer et al., 1974; Bedogne et al., 1993, 1999). A limited quantity (some thousands of carats) of Val Malenco demantoids have been cut, producing gemstones that are attractive but rarely exceed 2 or 3 ct (e.g., figure 1, right). Val Malenco demantoid was first documented by Cossa (1880), who studied a sample recovered by T. Taramelli in 1876. In the next century, Sigismund (1948) and Quareni and De Pieri (1966) described the morphology and some physical and chemical properties of this garnet. Subsequently, the demantoid was investigated by Bedogne and Pagano (1972), Amthauer et al. (1974), Stockton and Manson (1983), and Bedogne et al. (1993, 1999). Because some of these data are more than 20 years old, and some publications are in Italian, we prepared this review and update on the physical, chemical, and gemological properties of demantoid from Val Malenco. Note that demantoid—although commonly used as a trade or variety name—is not approved by the International Mineralogical Association as a mineral name (Nickel and Mandarino, 1987; O’Donoghue, 2006). However, for reasons of brevity and consistent with gemological convention, throughout this article we will use demantoid instead of andradite, variety demantoid.

The eclogite-bearing series of Isla Margarita, Venezuela: Geochemistry of metabasic lithologies in the La Rinconada and Juan Griego Groups

Abstract XRF analysis for major and minor elements in 94 samples of the metabasic La Rinconada Group and in 28 samples of metabasic lithologies in the predominantly metasedimentary, overlying Juan Griego Group of Isla Margarita, Venezuela, has been carried out to compare the bulk compositions of the two eclogite-bearing metabasic suites. The two units differ in the age and geological setting of the protolith. The La Rinconada Group has a monotonous basaltic character, but shows irregular variations in bulk composition which are interpreted to reflect variations in the type of protolith, ranging from basaltic lava flow to gabbroic, cumulitic dyke. As a whole, the La Rinconada Group is related to a source of tholeiitic composition in a MORB-type volcanic-hypabyssal environment. The Juan Griego metabasic rocks are compositionally more homogeneous and uniformly basaltic in their major elements, again reflecting a tholeiitic source and a MORB-type environment. Despite their contrasting geological setting, the two suites of metabasic rocks can be inferred to have had a similar source and have probably developed in a passive, “Atlantic”-type continental margin setting. We infer that the La Rinconada Group represents early oceanic crust later covered by the siliceous, continent-derived clastics of the Juan Griego Group, in which the minor basaltic intrusions represent the last stages of waning magmatic activity.

The ecologite-bearing metabasaltic sequence of Isla Margarita, Venezuela: A geochemical study

Abstract XRF analyses for twenty elements in 41 samples and RNAA for eight REE in ten samples have been carried out as part of a continuing project to elucidate the geological history of the La Rinconada Group metabasites of Margarita Island, Venezuela, through the study of continuous and discontinuous metamorphic reactions, the study of variations in amphibole composition and by means of a large-scale survey of bulk-rock geochemistry. All together, twelve sampling points taken along a continuous metamorphic profile were considered, with grade increasing from chlorite-bearing amphibolites to paragonite-bearing amphibolites to end up into amphibole-bearing eclogites. The entire section lies within the P — T field of conditions of the epidote-amphibolite facies. The La Rinconada Group as a whole has a basaltic character, although SiO 2 may reach as much as 54%. Despite being macroscopically monotonous the metabasites are chemically variable, with fractionation indices (i.e. FeO ∗ MgO ) varying by a factor of 5 from 0.48 to 2.66. There is no correlation between the zonation in the metamorphic mineral assemblages and the bulk-rock chemistry, so that this zonation must reflect only variations in the intensive parameters of metamorphism. A definitive distinction between MORB and IAT, or the PIA classification of Donnelly and Rogers (1978), is not yet possible on the basis of the present partial data set. The LIL elements, which are notoriously mobile, suggest an IAT affinity. However, the relatively immobile elements Ti, Cr, Zr, and Y favour MORB. REE patterns and enrichment factors, which are also considered fairly reliable criteria, display characteristics between those typical of MORB and PIA.

Equilibration conditions of eclogite lenses from Isla Margarita, Venezuela: Implications for the tectonic evolution of the metasedimentary Juan Griego Group

Abstract The constituent primary and secondary minerals in a suite of fifteen samples of eclogite, amphibole-eclogite and garnet-amphibolite, scattered as boudins and pods in metapelitic schists and gneisses on Isla Margarita, Venezuela, have been analysed by electron microprobe to augment existing bulk-rock chemical data. The large sample population available allows spurious effects of stoichiometric Fe3+ calculation procedures and bulk-rock influence to be recognised and eliminated. All samples belong to one population with a relatively homogeneous character. The equilibration temperatures are derived from Fe 2+ Mg fractionation between garnet and clinopyroxene and range from 525–650 °C. Jadeite contents of clinopyroxene and the persistence of minor stable albite constrain pressures to between 13 and 19 kbar. Combined with new evidence for high-pressure metamorphism in the enclosing metapelites and with existing data on an eclogitic metabasic unit exposed in north-eastern Isla Margarita (La Rinconada Group), these data show convincingly that the Margarita crustal block is and has been a relatively coherent unit ever since the entire complex suffered high-pressure metamorphism in Late Mesozoic times.

Petrology of the amphibolitized eclogites of Gorduno, Lepontine Alps, Switzerland

Abstract The amphibolite-facies gneisses outcropping west of Bellinzona, from Gorduno up to Alpe Arami, include boudins and layers of amphibolite with relics of eclogite. Well-preserved eclogites crop out at the margin of the garnet—peridotite of Alpe Arami and are included within the chlorite—peridotite bordering it. The chemical compositions of eclogites and relic-bearing amphibolites are so similar as to suggest isochemical retrograde metamorphism. However, a minor change can be traced, because the chemical affinity slowly trends from clearly tholeiitic to alkali-basaltic, while the trace elements still suggest derivation from “ocean-floor basalts”. Garnets in the eclogites are homogeneous almandine—pyrope (Alm 41 Pyr 36 Gro 23 ) but with increasing amphibolitization they become rimmed by more ferrous compositions (Alm 53 Pyr 27 Gro 20 ). Clinopyroxenes are omphacites with cores richer in Jd (Jd 35–40 ) than the rims (Jd 23–26 ). Symplectites contain diopside (Jd 6–8 ) in equilibrium with plagioclase (An 15–30 ). Jd-rich omphacites display the ordered P2/n symmetry, whereas Jd-poor omphacites are disordered C2/c. Primary brown amphiboles show extreme alumino-ferroanpargasitic compositions; green and colourless amphiboles are either hastingsites or Mg-hornblendes. The petrogenetic evolution of these rocks is bracketed between a maximum P — T condition of 720°C, 16 kbar and a minimum of 500°C, 4 kbar. The latter condition reflects declining stages of the amphibolite facies, later than the climax of the Lepontine phase. The former suggests derivation from oceanic mafic material subducted during the Eo-alpine phase, rather than from cognate mafic layers of upper-mantle origin. The geochemical evidence supports the possibility that they are neither mantle-derived nor in situ eclogites but tectonically emplaced foreign slices of oceanic material.

NEPHRITE JADE FROM VAL MALENCO , ITALY : R EVIEW AND UPDATE

Alpe Mastabia, in the Val Malenco district of northern Italy, has been a source of nephrite jade since the early 2000s. Twenty-one samples from this locality were investigated by classical gemological methods; X-ray powder diffraction, combined with quantitative phase analysis; scanning electron microscopy in combination with energy-dispersive spectrometry; electron microprobe analysis; mass spectrometry; and mid-infrared spectroscopy. From a mineralogical standpoint, this jade consists mainly of tremolite amphibole, with variable amounts of other constituents, especially calcite (up to approximately 30 wt.%), but also pyroxene, apatite, and sulfide minerals. Its pale green color is related to the low iron content of the tremolite amphibole, whereas the other minerals are responsible for different colors (cal cite for white, molybdenite and galena for gray). On the basis of minor and trace-element composition, we can classify this jade as dolomite-related nephrite (para-nephrite). Although new material could be recovered from this area, future production will probably be limited by access difficulties.

Petrogenesis of the eclogites from Soazza, Switzerland

Abstract The metabasite bodies interbedded in the kyanite—staurolite-subfacies gneisses of the Adula Nappe in the vicinity of Soazza (Mesolcina Valley) preserve the eclogite assemblage in their cores and grade into common amphibolite towards their margins. Garnets vary irregularly in composition from sample to sample. Omphacites included in garnets have Jd 32 , those occurring as large crystals Jd 45 , but intergranular laths again Jd 35 . They all display the ordered P2/n space group. Amphibole formed late in the eclogite assemblage is a pargasitic hornblende while those occurring in amphibolites are edenites or Mg-hornblendes. Garnet—clinopyroxene pairs equilibrated first at T = 675° C and P ≥ 15 kbar then at a constant T = 760° C under P conditions decreasing from 15 to 10 kbar; the amphibolite-facies assemblages equilibrated at ≈ 570° C , 6 kbar. Chemically, the eclogite relics plot within the field of “common” ( B -type) eclogites in high-grade metamorphic rocks. Magmatic trends are still indicated by their minor elements, and discrimination diagrams suggest that these eclogites were originally tholeiitic basalts of oceanic affinity. In spite of now being within a crustal sequence, the Soazza eclogites are believed to have been a part of an oceanic crust that underwent subduction and “Eo-alpine” metamorphism to be emplaced later, as disrupted solid wedges, during the Alpine tectonism and then recrystallized under Barrovian amphibolite-facies regional conditions during the “Lepontine” phase.

Cummingtonite in the amphibolites of the South-Alpine basement complex (upper Lake Como region, Italy): Its relationships with hornblende

SummaryCummingtonite and hornblende occur together in metamorphosed basic rocks interlayered with lower-amphibolite facies sillimanite-muscovite-bearing schists and gneisses in the upper Lake Como region, South-Alpine basement complex of the Alps. Textural, chemical and spectroscopic studies indicate that they do not represent equilibrium pairs, but that hornblende was overgrown by cummingtonite during a late stage of either temperature increase or, more likely, of isothermal pressure release, by a continuous reaction which also produced the reverse zoning of plagioclase.ZusammenfassungIm Gebiet des oberen Como-Sees, im südalpinen Basement-Komplex, kommen Cummingtonit und Hornblende zusammen in metamorphen, basischen Gesteinen mit Zwischenlagen von Sillimanit-Muskovit-führenden Schiefern und Gneisen der unteren Amphibolit-Fazies vor. Untersuchung der Texturen, der chemischen Zusammensetzung und Infrarot-Spektroskopie zeigen, daß diese nicht Gleichgewichts-Paare darstellen, sondern daß Hornblende während eines späten Stadiums, entweder bei zunehmender Temperatur, oder eher während isothermalen Druck-Nachlasses durch eine kontinuierliche Reaktion von Cummingtonit überwachsen wurde, die auch für die reverse Zonierung der Plagioklase verantwortlich ist.

Aquamarine from the Masino-Bregaglia Massif, Central Alps, Italy

GEMS & GEMOLOGY FALL 2009 he Masino-Bregaglia Massif (also known as the Bergell Massif) contains numerous granitic pegmatites hosting a remarkable variety of minerals—including aquamarine (figure 1)—that have attracted the interest of mineralogists and collectors since the late 18th century (e.g., Bedogné et al., 1995). Beryl from this area was initially mentioned by Repossi (1916) and Staub (1924). Subsequently, many other beryl occurrences were discovered in the massif. In their listing of the locations of historical beryl-bearing pegmatites, Hügi and Röwe (1970) indicated that the most important Italian deposits occurred in the areas of Val Bregaglia (Bregaglia Valley), Valle Mello, Cima di Zocca, Val Masino, Val Codera, and Alpe Vazzeda. In the 1970s, a limited amount of gem-quality aquamarine was recovered and cut from the Filone Silvana (Silvana dike), located in Val Codera. Masino-Bregaglia aquamarine crystals typically show a prismatic habit and measure several centimeters long, although some crystals attain ~15–20 cm in length. They range from light to dark greenish blue to blue or yellowgreen. Some gemand carving-quality aquamarine has been recovered, although the fact that most of the crystals contain numerous inclusions and fractures makes such material rare (Bedogné et al., 1995). To our knowledge, a gemological characterization of this aquamarine is lacking, except for the recent work of Caucia et al. (2008, in Italian). The present article builds on that work by supplying additional data obtained on a larger number of samples from four pegmatites in this area.