Switching bulk horizontal shortening directions and regional-scale partitioning of deformation during the Isan Orogeny in the Eastern Fold Belt, Mount Isa Inlier, Australia

Prolonged deformation for ca 150 Ma along the Eastern Fold Belt, Mount Isa Inlier, differentially partitioned into three distinct Mesoproterozoic tectonic domains. NW–SE-trending structures dominate the northern domain, whereas E–W- and N–S-trending structures dominate the central and southern domains, respectively.\ud \ud Changing the direction of bulk horizontal shortening from NE–SW to N–S to E–W shifted the locus of maximum tectono-metamorphic effect. This accounts for the different generations of structures preserved in these three domains. Overprinting relationships and geochronological data reveal a component of deformation partitioning in time as well as space.\ud \ud Rheological contrasts in the Soldiers Cap Group between a thick interlayered pelitic, psammitic and volcanic units on the one hand, and ca 1686 Ma, competent mafic intrusives and genetically related metasomatic albitite bodies present in its lower part, on the other, enhanced strain localisation during the long-lived Isan Orogeny (ca 1670–1500 Ma).\u

Changes in relative plate motion during the Isan orogeny (1670-1500 Ma) and implications for Pre-Rodinia reconstructions

Foliation inflexion/intersection axes (FIAs) preserved within porphyroblasts that grew throughout Isan orogenesis reveal significant anticlockwise changes in the direction of bulk horizontal shortening between 1670 and 1500 Ma from NE-SW, N-S, E-W to NW-SE. This implies an anticlockwise shift in relative plate motion with time during the Isan orogeny. Dating monazite grains amongst the axial planar foliations defining three of the four FIAs enabled an age for the periods of relative plate motion that produced these structures to be determined. Averaging the ages from monazite grains defining each FIA set revealed 1649±12 Ma for NE-SW shortening, 1645±7 Ma for N-S shortening, and 1591±10 Ma for that directed E-W. Inclusion trail asymmetries indicate shear senses of top to the SW for NW-SE FIAs and dominantly top to the N for E-W FIAs, reflecting thrusting towards the SW and N. No evidence for tectonism related to early NE-SW bulk horizontal shortening has previously been detected in the Mount Isa Inlier. Amalgamation of the Broken Hill and possibly the Gawler provinces with the Mount Isa province may have taken place during these periods of NE-SW and N-S-directed thrusting as the ages of tectonism are similar. Overlapping dates, tectonic, metamorphic, and metallogenic similarities between eastern Australia (Mount Isa and Broken Hill terranes) and the southwest part of Laurentia imply a most probable connection between both continental masses. Putting Australia in such position with respect to North America during the Late-Paleo-to-Mesoproterozoic time is consistent with the AUSWUS model of the Rodinia supercontinent

Foliation intersection axes (FIAs) preserved within porphyroblasts: resolving plan view orthogonal refolding, Eastern Fold Belt, Mt Isa Inlier

The relative timing of the sequence of events is problematic where deformation partitioning strongly separates zones dominated by folds that formed in different periods of orogeny. This is particularly the case where they form orthogonal to one another in plan view. A succession of foliation inflection/intersection axes (FIAs) preserved within porphyroblasts readily resolves the sequence. In situ monazite age dating of porphyroblasts preserving W–E and N–S-trending FIAs yielded ages of 1645 ± 7 and 1591 ± 10 Ma, respectively. Peaks in the distribution of the two FIA sets coincide with the trends of the axial planes of the regional folds. The succession of FIAs, as well as foliations in the matrix in the Soldiers Cap Group of the eastern Mount Isa Inlier, supported with geochronological data reveal W–E directed bulk horizontal shortening followed N–S bulk horizontal shortening. This is despite the apparent regional lack of curvature of the W–E-trending folds around those trending N–S. Furthermore, N–S and W–E-directed bulk shortening resulted in at least 2 cycles of subvertical and subhorizontal foliation development that define the W–E and N–S-trending FIAs

Radical changes in bulk shortening directions during orogenesis: significance for progressive development of regional folds and thrusts

Polyphase folding and metamorphism of rocks in the eastern Mount Isa Inlier resulted from four distinct orogenic cycles. These consist of NE–SW shortening (Oοο) from 1672 to ~1648 Ma, involving a succession of five phases of deformation, which was responsible for the development of the first generation NW–SE striking macroscopic folds, plus cleavages that are mainly only preserved in porphyroblasts. N–S shortening (O1) followed to 1600 Ma, involving at least three phases of deformation. This period of N–S directed orogenesis rotated large portions of the NW–SE trending regional folds into W–E trends and the dominance of locally well-preserved W–E trending sub-vertical matrix foliations as well as the intensification of foliation parallel to bedding. Subsequent W–E shortening (O2) from 1600 to~1515 Ma was accompanied by the development of N–S trending folds. These dominate the Mount Isa Inlier and where best developed involved the formation of at least three near orthogonal foliations. Where these large-scale folds regionally are weakly developed or not visible, only one sub-vertical foliation has developed. Finally NW–SE shortening (O3) produced locally developed SW–NE trending folds and weak crenulations until not, ~1500 Ma.\ud \ud The asymmetry of steep to gentle pitch changes in inclusion trails preserved in garnet porphyroblasts that developed during O00 indicates thrusting to the SW. Similar inclusion trail asymmetries in garnet, staurolite and andalusite porphyroblasts that developed in O1, indicate thrusting to the N confirming a change in thrust direction across the Wonga Duchess belt to the W. Formation of the Meerenurker roof thrust near Mount Isa by thrusting to the SW during O00 with subsequent development of the Kokkalukkanurker duplex below by thrusting to the S during O1 reduces the displacement required for the roof thrust from 234 to ~65 kms. Foliations preserved in O00 garnets are truncated by foliations associated with the development of O1 in the matrix. O1 garnet, staurolite and andalusite porphyroblasts preserve inclusion trails that are commonly truncated against a differentiated crenulation cleavage in the matrix that formed during O2. The inclusion trails entrapped within Oοο and O1 generated porphyroblasts reveal that the foliations that progressively developed in the matrix during these orogenies were composite fabrics resulting from the effects of multiple discrete orthogonal increments in each case. Numerous phases of sub-vertical foliation development due to horizontal bulk shortening were followed by phases of gently dipping foliation formation due to gravitational collapse that accompanied thrusting during Oοο and O1

Polymetamorphism accompanied switching in horizontal shortening during Isan Orogeny: example from the Eastern Fold Belt, Mount Isa Inlier, Australia

Mesoproterozoic low-P/high-T volcano-sedimentary rocks of the Soldiers Cap Group, the southeastern corner of Mount Isa Inlier, record a complex polymetamorphic history that accompanied four periods of bulk horizontal shortening directed NE–SW, N–S, W–E and NW–SE during the long-lived Isan Orogeny (~ 1650–1500 Ma). Low-P/high-T metamorphism (M₁) prevailed during a period of NE–SW bulk horizontal shortening as indicated by the early growth of cordierite porphyroblasts, which entrap monazite grains that gave an average age of 1649 ± 12 Ma. This was followed by medium-P/high-T (M₂) and high-P/high-T (M₃) metamorphisms that accompanied periods of N–S and W–E bulk horizontal shortening, respectively. Growth of first generation garnet, andalusite and staurolite porphyroblasts having an average age of 1645 ± 7 Ma identify the former, whereas growth of the 1591 ± 10 Ma second generation garnet, staurolite and andalusite porphyroblasts together with fewer kyanite porphyroblasts characterizes the latter.\ud \ud The sediments of the Soldiers Cap Group were deposited in a tectonic setting that has the characteristics of an intra-continental rift basin. The upper limit of the age of sedimentation is constrained by detrital zircons at 1654 ± 4 Ma suggesting that tectonism and metamorphism were either active during the final stage of the basin filling or immediate after deposition.\ud \ud Introduction of mafic dykes and sills at different stratigraphic levels over a wide time span was the major source of heat

Sequential deuteric and hydrothermal alterations in the Late Neoproterozoic Um Naggat rare metal-bearing granite, Central Eastern Desert, Egypt

The Late Neoproterozoic rare metal-bearing granite of Um Naggat pluton records deuteric and magmatic-hydrothermal fluid activities that culminated in overlapping sequential metasomatic alterations. The pluton is petrographically discriminated into subsolvus biotite, hypersolvus alkali-feldspar, and subsolvus albite granites. The residual fluid interacted and re-equilibrated with the mica and feldspars. Two phases of albitization, greisenization, chloritization and hematitization, and one phase of silicification, as well as microclinization are recognized. Compared to the alkali-feldspar and albite granites, the microclinization is restricted to the biotite granite, whereas greisenization and albitization increase in the former two granites and reaches their climax in the albite granite. Repeated contrasting acid-and alkaline-based alterations emphasize the fluctuation of the acidic-basic behavior of the fluid as a consequence of dominantly hydrogen-and alkali-exchange reactions