Interregional correlations for the Ross Orogen in the Ross Sea/Ross Ice Shelf sector of the Transantarctic Mountains: implications for the tectonic evolution of the palaeo-Pacific Antarctic margin of Gondwana (INTERTAM)

Rocks of the Ross Orogen of the Transantarctic Mountains (TAM) are generally interpreted to have evolved during a Neoproterozoic-early Paleozoic cycle of deposition and thermotectonic phases, spanning the breakup of Rodinia and the final consolidation of Gondwana. The main geological framework of the Ross Orogen may be subdivided along the c. 2000 km length of the TAM facing the Ross Sea / Ross Ice Shelf into three major segments, including Northern Victoria Land (NVL, from David Glacier to the Pacific end of the chain), southern Victoria Land (SVL, between Byrd and David Glaciers) and the central Transantarctic Mountains (CTAM, from Byrd Glacier to the Queen Maud Mountains). Voluminous granitoids (Granite Harbour Intrusive Complex - GHI) intruded as batholiths at c. 530-480 Ma, represent a unifying feature throughout the length of the TAM. But although the general tectonic history of the Ross Orogen is fairly well known within each of the major segments of the TAM, significant variations in lithostratigraphic, structural, and metamorphic patterns, as well as in granitoid geochemical affinity, are evident between the different segments. Until our knowledge of the relationships of the tectono-metamorphic histories and of the detailed chronology of the magmatic episodes between the segments is understood, a comprehensive tectonic model of the development of the Ross Orogen remains to be formulated. The areas targeted by this research were selected to allow a comparison of the tectono-metamorphic and igneous evolution within each of the three segments of the Ross orogenic belt in the Ross Sea/Ross Ice Shelf region. The correlation of litostratigraphic and structural development, as well as the main magmatic and metamophic episodes thoughout the wide area between the CTAM and NVL should give a deeper insight into critical aspects of the tectonic development of a significant portion of the Gondwana palaeo-Pacific margin. The field work on selected areas include collection of petrographic data at the macro scale and samples (stored in the Museo Nazionale dell’Antartide at Siena). Petrographic and mineral chemistry investigations as well as geochemical data for major and trace elements and isotopes have been carried out on selected rock units of the different areas. The main target of this research is the comparison of the tectono-metamorphic and magmatic evolution recorded in the pre-Devonian basement of the Ross orogenic belt in three seqments of the TAM. By these comparisons a correlation of units and orogenic phases between the NVL, SVL and CTAM segments of the orogen will be proposed. These final results will be achieved through a series of intermediate objectives, as follows: 1) description of lithotectonic features of metasedimentary rock assemblages, reconstruction of pre-metamorphic lithostratigraphic settings and of the structural development in each area, and relationships with the emplacement of local GHI units; 2) recognition of intrusive units and suites, reconstruction of the emplacement sequence and petrogenesis modelling; 3) acquisition of age data on both GHI and metamorphic rocks (Ar/Ar, U/Pb and Sm/Nd systematics) to constrain the temporal development of magmatic and tectono-metamorphic processes recorded in the metamorphic and GHI units; 4) reconstruction of P-T-D-t trajectories in the metamorphic units/complexes, and of temporal and structural relationships between the main metamorphic stages and the formation/emplacement of GHI in the three regions; 5) definition of regional correlations, and formulation of a tectonic model for the Ross orogen in the Ross Sea/Ross Ice Shelf sector of the TAM.The planned investigations are carried out through a multi-disciplinary approach, based on the integration and comparison among results obtained from coordinated structural, petrological and geochemical (major, trace, isotopic) studies in all targeted areas. The following will be accomplished: 1) evaluation of available data, rock collections, literature data, and field reports in order to select key-areas to be visited for their potential to explore the SVL/NVL boundary zone; compilation of data on 1:250000 thematic maps 2) field mapping to produce a 1:250000 map for each of the working regions and 1:50000 maps for local, selected areas; 3) reduction of structural data and selection of structurally significant samples for petrological studies finalized to the acquisition of microtextural and microchemical data through analytical SEM/EDS and WDS microscopy; 4) macrochemical analyses through XRF and AAS on selected samples from mapped GHI igneous rock units and metasedimentary rocks; 5) thermobarometric estimates based on both intra- and inter-crystalline cation exchange and on classic petrogenetic grids as defined by experimental data. These estimates will be compared with the thermometric isotopic ones based on anion exchange (O16/O18); 6) geochronological investigations for determination of the thermal history recorded in the metamorphic units (Sm/Nd, U/Pb, Ar/Ar), the timing of intrusive phases (U/Pb, Sm/Nd) and of the provenance age of detrital zircons (U/Pb) in metasedimentary units toward constraining the younger bound of their depositional ages; 7) modelling of reactions and microstructural-mineralogical evolution of rocks, reconstruction of the P-T-D-t paths and modelling of the thermal and geodynamic conditions during tectonic evolution.