The northern Ossa-Morena Cadomian batholith (Iberian Massif): magmatic arc origin and early evolution

Abstract

Diorites and related rocks in the Mérida area of northern Ossa-Morena (SW Iberia) are intrusive into Precambrian metavolcanic and metasedimentary sequences. Cumulate products from the H₂O-rich magmas are amphibole-rich gabbros to hornblendites. Major and trace element compositions, including Sr and Nd isotope data, allow the definition of a calc-alkaline series likely formed in relation to an immature arc setting. Crystallization of the intrusives has been established between ca. 570 and 580 Ma by U-Pb dating of constituent zircons. Garnet growth in dioritic rocks reflects a tectono–thermal overprint dated by Sm-Nd internal isochrons at around 555 Ma. Older Sm-Nd and Lu-Hf results between ca. 593 and 637 Ma on the same rocks suggest an earlier stage of regional metamorphism within the arc environment. The northern Ossa-Morena composite batholith and related metamorphic units have been tectonized and dismembered in the course of subsequent low-grade events during final stages of the Cadomian orogeny and the Variscan cycle. The units studied represent a well preserved segment of the arc region that evolved in Neoproterozoic times along the western border of Gondwana to conform the Cadomian–Avalonian basement of the Hercynian realm.

Appendix

Analytical techniques

Whole-rock major and trace element analysis: Major (wt.%) and trace element (μg/g) concentrations were measured by simultaneous ICP-AES at the University of Bilbao following procedures described by Cantagrel and Pin (1984) and Pin and Joannon (1997). Total iron is given as Fe₂O₃. Analytical uncertainty for trace elements is estimated as 10% or better.

Isotopic analysis: Isotope dilution thermal ionisation mass spectrometry (ID-TIMS) U–Pb analyses were performed at the CNRS, Clermont-Ferrand on the least magnetic (2° forward and side tilt at 2.2 A using a Frantz Isodynamic magnetic barrier separator), mechanically abraded (Krogh 1982), and crack-free zircon grains. Zircon dissolution, chemical separation of U and Pb, and isotope analyses were carried out according to methods described by Paquette and Pin (2001). The U and Pb isotopes were analysed on a VG Sector 54–30 mass spectrometer in multi-collector static mode. The isotopic ratios are corrected for mass discrimination (0.1±0.015% per amu for Pb and U), isotopic tracer contribution and analytical blanks: 10 pg for Pb and 1 pg for U. Initial common Pb is determined for each fraction in using the Stacey and Kramers (1975) two-step model. Data errors (2σ) of the zircon fractions and discordia lines were calculated using the PBDAT 1.24 and Isoplot/Ex 2.49b programs (Ludwig 1993, 2001).

Minerals for SHRIMP U-Pb isotopic analyses were hand-picked from defined sieve 0.35 mm and magnetic separator. Zircons were separated using a magnetic drum separator, a Frantz isodynamic separator and heavy liquids. The zircons selected are mounted in epoxy and polished until an equatorial section is reached, the zircons were studied under back-scatter electronic microscope with a cathodoluminiscence device at the Metallforschung Institut, ETH Zürich. The same mount is used for in situ ion-microprobe analyses (SHRIMP-I and II, Sensitive High Resolution Ion Micro Probes) carried out at ANU (Australian National University) in Canberra, Australia. For a full description of the ion-microprobe technique and data acquisition we refer to Compston et al. (1984, 1986). All U-Pb ages are referenced to a ²⁰⁶Pb/²³⁸U value of 0.0928 (equivalent to 572 Ma) for the standard zircon used from a pegmatite from Sri Lanka (SL-13). The U/Pb data are presented in a concordia diagram (Wetherill 1956, 1963), where ellipses are plotted with 1σ error. All the data presented in the concordia were corrected for the common lead using the ²⁰⁸Pb correction method.

Approximately 0·1 g of whole rock, spiked with ¹⁵⁰Nd-¹⁴⁹Sm mixed tracer solution, was used for Sm-Nd and Sr analyses. Sample dissolution, chemical separation of Sr, Sm and Nd and isotope analyses was performed following methods described by Pin and Santos Zalduegui (1997). Procedure blanks for Sr and Nd are typically below 50 pg. One garnet fraction of sample MER105 was leached prior to dissolution following a two-step procedure involving 1 ml of cold concentrated HF for 1 h followed by 7 N HNO₃ and 6 N HCl on a hot plate for 15 h. Sm-Nd and Sr isotope analyses were done at the University of the Basque Country using a Finnigan MAT262 on a static multicollection with a ¹⁴³Nd/¹⁴⁴Nd=0. 511844±13 (2σ, n =18) value for La Jolla standard and ⁸⁷Sr/⁸⁶Sr=0.71025±4 (2σ, n =33) for the Sr-standard NBS-987. Analytical uncertainty was estimated to be ± 0.2% on the ¹⁴⁷Sm/¹⁴⁴Nd ratio and 0.0016% on ¹⁴³Nd/¹⁴⁴Nd.

Hf isotopes were measured with a multiple-collector inductively coupled plasma mass spectrometer (MCICP-MS) at the Ecole Normale Supérieure de Lyon. Samples were processed using a new sample digestion and Lu-Hf separation scheme, involving fusion with a LiBO2 fluxing agent and one extraction chromatography column based on diamyl amylphoshonate (DAAP). Typical Hf and Lu blank/sample ratios were negligible. The ¹⁷⁶Lu decay constant of Scherer et al. (2001) was used in the Lu-Hf age calculations with Isoplot.

Microprobe analysis: Mineral analyses were done using an automatic Cameca SX50 microprobe equipped with three spectrometers at the University of Oviedo (Spain). Operating parameters included a 10-s-counting time (peak), a c. 10 nA beam current and a 15 kV accelerating voltage. Calibration was against BRGM (French Geological Survey) standard minerals, and the ZAF correction procedure was used. Table 1 presents a selection of analytical data with additional details on structural formulas and Fe³⁺ estimates.