Formation of Amphibole and Clinozoisite–Epidote in Eclogite owing to Fluid Infiltration during Exhumation in a Subduction Channel

JOURNAL OF PETROLOGY VOLUME 53 NUMBER 10 PAGES 1969^1998 2012 doi:10.1093/petrology/egs040 Formation of Amphibole and Clinozoisite^Epidote in Eclogite owing to Fluid Infiltration during Exhumation in a Subduction Channel H.-J. MASSONNE* INSTITUT FU«R MINERALOGIE UND KRISTALLCHEMIE, UNIVERSITA«T STUTTGART, AZENBERGSTR. 18, D-70174 STUTTGART, GERMANY RECEIVED FEBRUARY 28, 2011; ACCEPTED MAY 18, 2012 ADVANCE ACCESS PUBLICATION JULY 12, 2012 Two eclogites, one from the southern Dabie Shan, China, and the other from the western Erzgebirge, Germany, were studied because of amphibole and clinozoisite^epidote formation either as typical porphyroblasts or as oriented fine-grained matrix minerals. A nearly isobaric prograde path was derived for the eclogite from the Erzgebirge, mainly on the basis of garnet zonation and the reconstruction of P^T pseudosections in the system SiO2^TiO2^ Al2O3^MgO^MnO^FeO^CaO^Na2O^K2O^H2O^O2. Peak P^T conditions were 2·8 GPa and 6708C. The retrograde path shows significant cooling during pressure release. In contrast, the Dabie Shan eclogite contains sparse Mg-rich garnet cores pointing to a granulite stage. Subsequently, peak-pressure conditions of 3·5^4·0 GPa were reached at 6208C, followed by a retrograde path that was initially nearly isothermal and then similar to that of the Erzgebirge eclogite. Most of the amphibole and clinozoisite^ epidote grew during early retrogression in the pressure interval 1·8^2·3 GPa (Erzgebirge) or 2·4^2·7 GPa (Dabie Shan) owing to infiltration of hydrous fluids amounting to 1wt % and 0·5 wt % H2O, respectively. As a result of this hydration omphacite was partially decomposed, but the remaining omphacite demonstrates that infiltration of H2O essentially ceased at 1·8 GPa (Erzgebirge) or 2·4 GPa (Dabie Shan). The fluid infiltration and hence the growth of amphibole and clinozoisite^epidote is proposed to have occurred in a deep-seated subduction channel during ascent of eclogitic material. Subsequently, the eclogites were captured between colliding continental plates and further exhumed in an exhumation channel. KEY WORDS: eclogite; Saxonian Erzgebirge; Dabie Shan; amphibole; clinozoisite^epidote; subduction channel *Corresponding author. Telephone: þ49-711-68581225. þ49-711-68581222. E-mail: ß The Author 2012. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@ oup.com Fax: I N T RO D U C T I O N Eclogite occurs in virtually all young orogens that were formed by continent^continent collision. In these orogens this rock type, when formed at low to medium temperatures (see Carswell, 1990), often contains amphibole and clinozoisite^epidote. Although often only casually mentioned in rock descriptions these minerals tend to form porphyroblasts in eclogite. This feature is supported by the statement of Liou et al. (2008) that ‘porphyroblastic sodic amphibole is also common as a primary phase in eclogites of many continent collisional orogens’. Such porphyroblasts have long been recognized, as they sometimes form centimetre-sized, idiomorphic crystals. For instance, zoisite was named after Baron Sigmund Zois by Abraham Gottlob Werner in 1805 (Faninger, 1991). The type locality Prickler Halt is a pegmatoid in eclogite of the Saualpe crystalline complex in Carinthia, Austria. The amphibole carinthine was also first described from an eclogite body of the Saualpe at that time (Faninger, 1991). Even the name eclogite, going back to Rene¤-Just Hau«y (Godard, 2001), was probably assigned to these (clino)zoisite and amphibole-bearing bodies (Mottana et al., 1968). The precise chemical compositions of these amphibole and JOURNAL OF PETROLOGY VOLUME 53 clinozoisite^epidote porphyroblasts were the subject of a number of earlier studies (e.g. Miller, 1974; Smulikowski & Smulikowski, 1985; Messiga et al., 1991) but were also more recently addressed (e.g. Su et al., 2009; Tagiri et al., 2010; Wilke et al., 2010a). These studies include the aforementioned eclogites from the Saualpe type locality (Miller, 1990). Nevertheless, the precise conditions of the formation of amphibole and clinozoisite^epidote (occasionally also zoisite) porphyroblasts in eclogite have only rarely been addressed; for instance, by Massonne (1992), Zack et al. (2001), Garcia¤-Casco et al. (2002) and Massonne & Kopp (2005). According to the results of those workers, pressure^temperature (P^T) conditions around 2 GPa and 6008C seem to be typical for the formation of these porphyroblasts. To obtain a more extended dataset on such conditions and, thus, to better understand the reason for the growth of these minerals, examples of eclogites with considerable amounts of amphibole and clinozoisite^ epidote, formed at eclogite-facies conditions, were chosen for a detailed study. Furthermore, it was the aim of this study to recognize the role of water and to quantify the water budget relevant to the formation of these minerals in eclogite. For such quantification, the use of P^T pseudosections, calculated with the PERPLE_X software, has turned out to be very practical (e.g. Massonne & Willner, 2008; Willner et al. 2011). However, other software is also capable of such calculations (see Konrad-Schmolke et al., 2011). A selection of suitable material for the envisaged study was made based on the author’s collection of fresh and previously studied amphibole- and clinozoisite^epidotebearing eclogites from continent^continent collisional orogens, such as the Norwegian Caledonides, the West and Mid-European Variscides, the Dabie^Sulu belt in China, and the European Alps. Two samples were chosen for the following reasons. (1) Material from the western Erzgebirge, Mid-European Variscides, was chosen as it contains considerable amounts of oriented and finegrained amphibole and epidote^clinozoisite formed at eclogite-facies conditions. It can be demonstrated that porphyroblastic growth is not a necessary feature for the formation of hydrous minerals during a late eclogitic stage, although in the vicinity of the selected sample undeformed eclogite bodies occur, which often display porphyroblasts of these minerals (see Massonne & Kopp, 2005; Faryad et al., 2011). (2) Fresh eclogite from the southern Dabie Shan, China, was selected because it contains abundant amphibole and clinozoisite^epidote porphyroblasts in a low-variance assemblage. In addition, the chosen sample shows unusual, Mg-rich garnet core compositions. It will be demonstrated below that these cores result from a high-T pre-eclogitic metamorphism. Thus, the two selected rocks have a completely different early metamorphic history but, nevertheless, demonstrate NUMBER 10 OCTOBER 2012 a common metamorphic process related to the late formation of amphibole and clinozoisite^epidote. The geodynamic setting of a subduction channel is proposed for this process as part of a geodynamic scenario that leads to the appearance of amphibole- and clinozo (...truncated)