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Anticlockwise P–T Path of Granulites from the Monte Castelo Gabbro (O´ Rdenes Complex, NW Spain)

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Anticlockwise P–T Path of Granulites from the Monte Castelo Gabbro (O´ Rdenes Complex, NW Spain) JOURNAL OF PETROLOGY VOLUME 44 NUMBER 2 PAGES 305–327 2003 Anticlockwise P–T Path of Granulites from the Monte Castelo Gabbro (O´ rdenes Complex, NW Spain) JACOBO ABATI1∗, RICARDO ARENAS1, JOSE´ RAMO´ N MARTI´NEZ CATALA´ N2 AND FLORENTINO DI´AZ GARCI´A3 1DEPARTAMENTO DE PETROLOGI´A Y GEOQUI´MICA, FACULTAD DE C.C. GEOLO´ GICAS, UNIVERSIDAD COMPLUTENSE, 28040 MADRID, SPAIN 2DEPARTAMENTO DE GEOLOGI´A, UNIVERSIDAD DE SALAMANCA, 37008 SALAMANCA, SPAIN 3DEPARTAMENTO DE GEOLOGI´A, UNIVERSIDAD DE OVIEDO, 33005 OVIEDO, SPAIN RECEIVED JULY 1, 2001; REVISED TYPESCRIPT ACCEPTED AUGUST 13, 2002 The study of mafic and aluminous granulites from the Monte INTRODUCTION ´ Castelo Gabbro (Ordenes Complex, NW Spain) reveals an anti- Thermobarometric estimations in high-T metamorphic clockwise P–T path that we interpret as related to the tectonothermal rocks are subject to important uncertainties, because the activity in a magmatic arc, probably an island arc. The P–T path chemical composition of metamorphic peak minerals is was obtained after a detailed study of the textural relationships and frequently modified by retrograde diffusion phenomena, mineral assemblage succession in the aluminous granulites, and especially with respect to Fe–Mg exchanges (Spear & comparing these with an appropriate petrogenetic grid. Additional Florence, 1992; Carlson & Schwarze, 1997). Hence, in thermobarometry was also performed. The granulites are highly many cases, an adequate textural interpretation and heterogeneous, with distinct compositional domains that may alternate deduction of the reaction sequence, in combination with even at thin-section scale. Garnets are generally idiomorphic to the use of an appropriate petrogenetic grid, is a more subidiomorphic, and in certain domains of the aluminous granulites reliable way to constrain the P–T path. Moreover, this they show overgrowths forming xenomorphic coronas around a more approach facilitates the selection of relevant mineral or less idiomorphic core. Both types of garnets show significant Ca assemblages and compositions for thermobarometry, and enrichment at the crystal rims, which, together with the other the comparison of the obtained quantitative P–T values mineralogical and textural characteristics, is compatible with a with the petrogenetic grid makes it possible to check pressure increase with low T variation. P–T estimations indicate their mutual compatibility. ° > a peak of T >800 C and P 9·5 kbar, attained after a This approach has been followed in this investigation, significant increase in pressure that took place at high temperatures devoted to the study of mafic and aluminous granulites (in the sillimanite field). We suggest that this kind of trajectory, from the Monte Castelo Gabbro, located in the up- probably anticlockwise, is compatible only with a terrane heated by permost unit of the O´ rdenes Complex (NW Spain). The an intense magmatic activity after or during tectonic crustal thickening aluminous granulites show a complex textural evolution, (magmatic injection at the base and/or into the crust), which is which can be followed in terms of the temporal succession characteristic of magmatic arcs. of mineral associations. The textures are rich and varied, which permits the reconstruction of equilibrium mineral assemblages for distinct periods during metamorphic evolution and, in some cases, allows us to distinguish KEY WORDS: granulites; garnet zoning; anticlockwise P–T path; Iberian which reactions are implicated in the progress from one Massif; NW Spain assemblage to the next. The coexistence of the different ∗Corresponding author. Telephone: 34-91-3945013. Fax: 34-91- 5442535. E-mail: [email protected] Oxford University Press 2003 JOURNAL OF PETROLOGY VOLUME 44 NUMBER 2 FEBRUARY 2003 mineral assemblages in the aluminous granulites is pos- to the Variscan orogenic cycle. The Early Ordovician sible mainly because of their strong heterogeneity, with event has been dated by U–Pb analyses of zircon, mon- distinct textural and compositional domains that may azite, titanite and rutile at >500–480 Ma (Abati et al., coexist in the same thin section. The mafic granulites do 1999; Ferna´ndez Sua´rez et al., 2002), and is probably not exhibit such textural variation, but it is still possible responsible for the main metamorphism observed in both to recognize parts of the metamorphic stages evidenced the HP–HT and the IP units. It can be inferred from in the aluminous granulites. the above studies that the HP metamorphism took place Applying the petrogenic grid of Spear et al. (1999) to during the Early Ordovician cycle (Ferna´ndez-Sua´rez et the aluminous granulites, we have constructed a fragment al., 2002), and the same age has been obtained for the of the P–T path. In addition, we performed thermo- IP metamorphism that formed the granulites of the units barometry calculations using the mineral compositions described in this paper (Abati et al., 1999). Geo- of the appropriate assemblages and the TWQ method chronological evidence for subsequent tectonothermal of Berman (1991). The results of the thermobarometry activity in these units is generally restricted to zones are slightly different from the results obtained using the of intense Variscan reworking (essentially shear zones). petrogenetic grid. However, we believe that they are Within these zones, monazite, titanite and rutile isotopic essentially compatible given the uncertainties in both the data indicate that a second metamorphic episode took thermobarometry calculations and the assemblages in place between >400 and 380 Ma, i.e. during the the petrogenetic grid. We conclude by interpreting the Variscan cycle. P–T path in a tectonic framework and by analysing its The IP upper units occupy most of the O´ rdenes implications for the regional geology of the O´ rdenes Complex (Fig. 1). They include a very thick sequence of Complex. terrigenous metasediments, and large bodies of am- phibolite, augengneiss and metagabbro. Metamorphic grade ranges from greenschist facies in the structurally uppermost part to IP granulite facies in the lower. The GEOLOGICAL SETTING lowermost IP unit in the NW sector of the complex is a The O´ rdenes Complex is one of the allochthonous struc- large, rounded metaigneous massif, the Monte Castelo tures of the NW Iberian Massif (Fig. 1), whose em- Gabbro (MCG, Fig. 2). The MCG is tholeiitic in char- placement in the axial part of the orogen took place acter (Andonaegui et al., 2002), and has been dated at during the Variscan continental collision. These struc- 499 ± 3 Ma (Abati et al., 1999; U–Pb in zircons). The tures consist of a stack of thrust sheets that are preserved intrusion is composed of fine- to medium-grained two- in late, open synforms. Three main tectonostratigraphic pyroxene gabbro, with textures ranging from granular ensembles have been identified in the O´ rdenes Complex, to intergranular and ophitic, and with an igneous mineral which are essentially the same as those recognized in the assemblage consisting of clinopyroxene, orthopyroxene, other allochthonous complexes of northwestern Iberia. plagioclase, scarce olivine, biotite and hornblende. Il- These are, from bottom to top, the basal, ophiolitic and menite, titanite, and zircon are the main accessory min- upper units. Assuming that the ophiolites represent a erals. The presence of olivine and the common ophitic collisional suture, the underlying basal units are in- textures point to a relatively shallow depth of em- terpreted as belonging to the margin of Gondwana and placement. the upper units are considered far-travelled terranes Although most of the massif preserves its igneous of yet unconstrained provenance (Arenas et al., 1986; texture and mineral assemblages, mafic granulites were Martı´nez Catala´n et al., 1997, 1999). The general geo- formed in localized shear zones in the basal part of the logical features and the evolution of the allochthonous MCG, whereas aluminous granulites developed during complexes have been the subject of numerous studies the high-temperature metamorphism of aluminous (prob- [for a comprehensive review, see Martı´nez Catala´n et al. ably metapelitic) enclaves inside the gabbro (Fig. 2). (1999) and references therein]. Monazites separated from the aluminous granulites yiel- The upper units of the O´ rdenes Complex can be ded a U–Pb age of 496 Ma for the metamorphism (Abati subdivided into high-pressure and high-temperature et al., 1999). (HP–HT) units, below, and intermediate-pressure (IP) units, structurally above, with the Monte Castelo Gabbro being one of the IP upper units. The upper units show the FIELD RELATIONSHIPS OF THE complexities typical of terranes that have been involved in more than one orogenic cycle. These rocks record a GRANULITES structural and metamorphic evolution related to an Early The aluminous granulites are found as enclaves dispersed Ordovician orogenic event, which is locally overprinted in the MCG. They appear as bodies of variable size that by structures, fabrics and metamorphic features related can reach >1 km in length and several hundreds of 306 ABATI et al. GRANULITES OF O´ RDENES COMPLEX, SPAIN Fig. 1. Geological sketch and cross-section of the O´ rdenes Complex showing the various units. metres in thickness, usually trending NW–SE (Fig. 2) 1967) to interpret the metamorphism of the enclaves and showing a high-T foliation with much evidence of as the result of contact metamorphism, or of different partial melting, such as leucosomes oriented parallel to contamination reactions between
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