FIAs(Foliation Intersection/Inflection Axes) Preserved in Porphyroblasts,the DNA of Deformation:A Solution to the Puzzle of Deformation and Metamorphism in the Colorado,Rocky Mountains,USA

FIAs have been used extensively for more than a decade to unravel deformation and metamorphic puzzles. Orogenic processes developing early during the history or orogenesis challenge scientists because compositional layering in rocks always reactivates where multiple deformations have occurred, leaving little evidence of the history of foliation development preserved in the matrix. The foothills of the Rocky Mountains in Colorado, USA contain a succession of four FIA sets （trends） that would not have been distinguishable if porphyroblasts had not grown during the multiple deformation events that affected these rocks or if they had rotated as these events took place. They reveal that both the partitioning of deformation and the location of isograds changed significantly as the deformation proceeded.

Resolving the Routine Presence of Kyanite, Andalusite and Sillimanite across a Region using Foliation Intersection/Inflection Axes Preserved in Porphyroblasts, Petrographic Observations and Thermobarometry

Constraints from P-T pseudosections （MnNCKFMASH system）, foliation intersection/ inflection axes preserved in porphyroblasts （FIAs）, mineral assemblages and textural relationships for rocks containing all three Al2 SiO5 polymorphs indicate a kyanite→ andalusite→ sillimanite sequential formation at different times rather than stable coexistence at the Al2SiO5 triple point. All three Al2SiO5 polymorphs grew in the Chl, Bt, Ms, Grt, St, Pl and Crd bearing Ordovician Clayhole Schist in Balcooma, northeastern Australia separately along a looped P-T-t-D path that swaps from clockwise to anticlockwise in the tectono-metamorphic history of the region. Kyanite grew during crustal thickening in an Early Silurian Orogenic event followed by decompression/heating, andalusite and fibrolitic sillimanite growth during Early Devonian exhumation.

Chemical U-Th-Pb Monazite Dating of Deformations versus Pluton Emplacement and the Proterozoic History of the Arkansas River Region,Colorado,USA

Five lengthy periods involving multiple phases of cordierite and andalusite growth were revealed by detailed studies of foliation inflection/intersection axes （FIA） preserved in porphyroblasts in schists from the Arkansas River region in Colorado, USA. The regionally consistent character of the succession of five different FIA trends enabled the relative timing of each FIA with respect to the next to be determined. The FIA succession from first to last is： FIA 1 trending W-E, FIA 2 trending SSW- NNE, FIA 3 trending NNW-SSE, FIA 4 trending NW-SE and FIA 5 trending SW-NE. For four of the FIA sets, samples were found containing monazite grains preserved as inclusions. These were dated on an electron microprobe. The ages obtained concur exactly with the FIA succession, with FIA 1 at 1506±15 Ma, FIA 2 at 1467±23 Ma, FIA 3 at 1425±18 Ma, FIA 4 not dated and FIA 5 at 1366±20 Ma. These ages are directly reflected in a succession of plutons in the surrounding region dated by other isotopic approaches, suggesting that deformation, metamorphism and pluton emplacement occurred together episodically, but effectively continuously, for some 140 Ma.

Survival Conditions of Folding in Different Depth During Orogenesis-Deformation in Texas Creek and Chester Dome, USA

A succession of 5 FIA trends（foliation intersection or inflection axes in porphyroblasts） preserved in high temperature-low pressure regime PreCambrian rocks in the Texas Creek, Arkansas River region reflected by the fold axial plane traces and schistosity data in this region. Similar fold axial plane trace data measured in Palaeozoic rocks in Chester Dome, Vermont, which is high temperature to medium pressure regime, only preserve the effects of the youngest FIAs of the all 5 FIA sets that obtained in this region. The other three FIA sets have no equivalent fold axial planes. This difference from shallow to deeper orogenic regimes reflects decreasing competency at greater pressure with collapse and unfolding of earlier formed folds. The greater overlying load of rocks has tended to flatten all but the very largest early-formed structures, preserving only those folds that were more recently developed.

Changes in Relative Plate Motion during the Isan Orogeny(1670-1500Ma) 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.