The Hyland River schist-gneiss belt is an elongate, partly fault-bounded regional-metamorphic high (approximately 2000 km(2)) in the northern Canadian Cordillera. It includes a migmatitic core flanked by schist and phyllite and hosts numerous large mid-Cretaceous intrusions. Regional-metamorphic grade increases progressively across unfaulted flanks from the chlorite zone to the cordierite + K-feldspar zone over a structural thickness of similar to 3.5-4 km. An early stage of regional metamorphism (M-2) produced garnet and staurolite (+/- sillimanite)-bearing schists. This was followed by low-pressure, high-temperature (LP-HT) regional metamorphism (M-3), which resulted in partial melting of deep structural levels. Regional (M2-3) isograds were folded (F-4) prior to the emplacement of late- to post-tectonic batholiths. Upright F-4 folds produced kilometre-scale structural relief, with the result that contact aureoles were developed across steep regional-metamorphic gradients. We document the timescales involved in the transition from regional LP-HT to contact metamorphism by integrating mineralogical and microstructural observations with high resolution U-Pb TIMS geochronological data from variably deformed intrusions. Felsic-intermediate magmatism within the schist-gneiss belt extended over a period of >= 7 My. LP-HT regional metamorphism (M-3) overlapped with syntectonic leucogranite and pegmatite intrusions (similar to 112-106.9 Ma) but took place prior to the emplacement of large, late- to post-kinematic batholiths of granite and granodiorite (106.2-105.1 Ma). Folding of regional isograds took place during the short interval of time between peak M-3 metamorphism and formation of the batholiths (< 1 My). The small age difference between regional LP-HT and contact metamorphism attests to a common magmatic cause and illustrates the short timescales over which overprinting relationships can develop in such settings.
The North Tianshan Belt in the southwestern Central Asian Orogenic Belt (CAOB) provides unique insights into crustal evolution and geodynamic processes of accretionary orogeny. Here, we present new structural, thermobarometric and geochronological data from the Xiaopu Metamorphic Complex (XMC) in the eastern North Tianshan. Structural analysis indicates that an initial shortening phase (D1), which produced NW-SE-striking folds, was followed by a sinistral transtension (D2). The latter is characterized by syn-kinematic crystallization of peak mineral assemblages, suggesting an inverted Buchan-type metamorphic sequence. Subsequent strike-perpendicular extension (D3) likely facilitated the exhumation of the XMC. Phase modelling and geothermobarometry show a progressive increase in peak temperature from the andalusite-staurolite zone (similar to 580 degrees C) to the sillimanite-migmatite zone (similar to 690 degrees C) at nearly constant pressure (similar to 4 kbar), indicating a high-temperature and low-pressure (HT-LP) thermal regime consistent with an extensional setting. U-Pb dating on zircon, monazite, and apatite indicates that D2 occurred at similar to 330-310 Ma, coeval with the HT-LP metamorphism (320-310 Ma), and that D3 occurred at similar to 285 Ma. Biotite Ar-40/Ar-39 dating confirms syn-/post-D3 cooling (298-275 Ma). We suggest that magmatic intrusion controlled by transtension at the mid-crustal level and asymmetric exhumation likely formed the inverted Buchan-type metamorphic sequence. Our new results, together with regional data, reveal the late Palaeozoic tectono-metamorphic evolution of the southwestern CAOB, involving arc-arc accretion, back-arc transtension, and post-orogenic extension. These processes governed the tectonic transition from shortening to extension in the mid-crust of arc systems and induced magmatic intrusion and crustal exhumation during the final stages of accretionary orogeny.
Phase equilibrium modelling of metamorphic rocks is subject to multiple, superimposed sources of uncertainty that affect pressure-temperature ( P$$ P $$ - T$$ T $$ ) estimates, making it difficult to independently distinguish and evaluate the contribution of each source. Here, we summarise the principal sources of uncertainty (geological, analytical, parameter and model) and identify those that can be robustly quantified within current modelling frameworks, focusing on uncertainty arising from natural petrological variability and measurement uncertainty in the bulk rock composition and mineral data (compositions and modal proportions). To explore these effects on P$$ P $$ - T$$ T $$ estimates under an equilibrium scenario we construct a synthetic system based on the median worldwide pelite composition of Forshaw and Pattison (2023), in which individual sources of uncertainty are isolated and perturbed independently. We perturb either the bulk composition, from which forward models and predicted mineral data are generated through phase equilibrium modelling, the predicted mineral data, or both simultaneously. The grid-search inversion workflow of Mackay-Champion and Cawood (2026) is used to quantify the effects of the perturbed datasets on P$$ P $$ - T$$ T $$ estimates across contact- and regionally metamorphosed subsolidus conditions. Lower-grade conditions yield nearly twice the P$$ P $$ - T$$ T $$ uncertainty of higher-grade conditions. Uncertainty, presented at 1 sigma$$ \sigma $$ , decreases with increasing metamorphic grade: from 25 degrees C to 30 degrees C and 1.2-1.8 kbar at lower-temperature conditions (400 degrees C-550 degrees C) to 16 degrees C and 0.8 kbar at higher-temperature conditions (550 degrees C-700 degrees C). Temperature uncertainty is largest (29 degrees C) at lower pressure conditions (1-3.5 kbar) and smallest (16 degrees C) at higher pressure conditions ( >=$$ \ge $$ 6.5 kbar), whereas pressure uncertainty remains broadly constant (0.9-1.1 kbar) across all pressure conditions (1-12 kbar). The source of uncertainty and choice of input variables strongly influence P$$ P $$ - T$$ T $$ estimates, with uncertainty in the bulk composition exerting the dominant control, and inversions using only mineral modal proportion variables typically yielding approximately double the uncertainty of those using only mineral composition variables. These results provide an equilibrium-defined baseline for P$$ P $$ - T$$ T $$ uncertainty arising from uncertainties in mineral data and bulk composition, although tectonometamorphic and kinetic controls may amplify or reduce this uncertainty.
Rifted continental fragments provide critical records of metamorphic conditions and processes associated with the transition from continental extension to subsequent convergence along active margins. In particular, the shallower crustal sections of rifted crust that later collide with island arcs are comparatively less well preserved and, consequently, remain understudied relative to the deeper crustal sections. Furthermore, distinguishing metamorphic signatures associated with rifting from those related to subsequent collision remains challenging due to overprinting of earlier mineral assemblages and structures. In this work, we investigate the Romblon Metamorphic Complex (RMC) in the Philippines, an exposed fragment of continental crust located at the easternmost edge of the Palawan-Mindoro Microcontinental Block (PCB), which records both extensional and collisional processes along the western Pacific margin. The RMC consists mainly of interlayered metapelites and metapsammites exposed in the three main islands of the Romblon Island Group (RIG), namely Tablas, Romblon and Sibuyan. We integrate petrography, Raman spectroscopy of carbonaceous material, mineral thermometry, phase equilibrium modelling and zircon U-Pb geochronology to reconstruct its pressure-temperature-deformation (P-T-D) evolution. The results reveal two distinct metamorphic stages. Stage 1 represents a regional greenschist-facies event (similar to 520 degrees C, up to 6.0 kbar) associated with extension-related deformation, preserved by sedimentary layering and structures such as boudinage. Stage 2 records localised amphibolite-facies metamorphism (similar to 610 degrees C, 3.2-4.7 kbar) preserved proximal to the contact between the RMC and the overlying Sibuyan Ophiolite Complex (SOC), reflecting later tectonic modification during arc-continent collision. Detrital zircon U-Pb ages from metasedimentary rocks indicate a predominantly South China affinity, with youngest zircon populations of Early Eocene age (similar to 55.1 +/- 1.4 Ma). This work demonstrates that rifted continental fragments, such as the RMC, offer insights into the effects of continental rifting and subsequent tectonic reworking during collision. In particular, the two-stage metamorphic evolution preserved in the RMC metasedimentary rocks provides empirical constraints on the thermal conditions of shallow, low pressure (i.e., <6 kbar) crustal regions during rifting (= 26 degrees C/km) and subsequent arc-continent collision (= 43-63 degrees C/km). Our detailed investigation of the RMC further highlights the importance of integrating P-T-D analyses with geochronological approaches to distinguish imprints of multiple tectonic events preserved in metamorphic terranes.
