Search

Structures and alteration associated with mineralizing, pre-anatectic hydrothermal alteration at the Challenger gold mine in South Australia strongly influenced and preferentially focused subsequent metamorphism, partial melting and deformation. This event resulted in efficient deformation-enhanced melt extraction from the host rocks immediately adjacent to mineralization, leaving restitic garnet, cordierite, Kfeldspar rich mineral assemblages. Progressive reintegration of silicate melt into the highly restitic proximal gneiss bulk composition, using mineral equilibrium modelling in the system NCKFMASHTO, indicates that hydrothermal alteration stabilized a higher proportion of hydrous minerals leading to increased fertility and thus melting in the proximal gneiss. Further modelling at lower temperatures provides an insight into mineral assemblages that may have been associated with mineralization. Temperature-composition modelling provides evidence that composite gold-sulphide melts were armoured by quartz and as such were immobile during peak metamorphism. At Challenger, melt permeability of the mineralized zones was enhanced via the formation of a structural fabric, folding, boudinage, fracturing and the formation of shear zones. Giving consideration to the relationships and orientations of these structures, a model for preferential deformation and melt migration within the mineralized shear zone under transpression is developed, where melt is essentially pumped out of the system parallel to the maximum principal elongation direction. doi: 10.1144/0016-76492010-008.

The Entia Gneiss Complex represents the mid‐crustal core of the intracontinental Alice Springs Orogen. Located in the Harts Range, central Australia, it is characterized by the development of a domal structure including two sub‐domes separated by a steeply dipping median high‐strain zone. Dominantly, orthogneiss basement crops out through a structurally overlying cover sequence represented by the Harts Range Group and records evidence of hydration, recrystallization, and partial melting of precursor Paleoproterozoic granulite facies assemblages at amphibolite facies conditions. The structurally lowest parts of the Harts Range Group were metamorphosed to peak conditions of 10.5 kbar and ~880°C during rift‐related magmatism at 480–460 Ma, immediately prior to the onset of the Alice Springs Orogeny at 450–300 Ma. By contrast, the underlying Entia Gneiss Complex records widespread metamorphism and wet melting at upper amphibolite facies conditions. Phase equilibrium modelling and in situ LA–ICP–MS geochronology of rare, low‐variance kyanite–garnet‐bearing metapelites indicate that maximum P–T conditions of ~9 kbar and ~680°C were reached between c. 360–330 Ma, demonstrating a distinctive metamorphic and temporal evolution relative to the overlying Harts Range Group that can be linked to rheological stratification. Based on the integration of our results with existing monazite, zircon, and titanite geochronology, we interpret doming of the Entia Gneiss Complex to have involved compressive ascent of rheologically weakened crust below a region of extending upper crust near the termination of the Alice Springs Orogeny. In this way, the Harts Range Group represents a cooled, locally extending thrust sheet over ductile basement quasi‐concurrent with doming. Texturally‐late sillimanite combined with increasing Y content in monazite indicates high‐temperature, kyanite‐grade metamorphism was closely followed by decompression (3–4 kbar drop from peak conditions) and rapid cooling below 600°C during emplacement of the Entia Gneiss Complex at shallower crustal levels. The findings of this study highlight the feedback between hydration, retrogression, rheological weakening, and strain accommodation, thus allowing better evaluation of the thermomechanical history of gneiss dome formation within a narrow (<80 km wide) preconditioned intracontinental corridor. Highlights: First modern P–T–t framework for gneiss dome formation in the Entia Gneiss Complex, central Australia.The onset of metamorphism, partial melting and ductile flow at c. 360 Ma was catalysed by hydration of granulite facies basement during rift inversion.Rapid exhumation of partially molten crust from depths of >20 km is marked by fluid‐mediated resetting of monazite down to c. 310 Ma.Rheological stratification enabled and accelerated exhumation of the highest‐grade corridor of a narrow intracontinental orogen. [ABSTRACT FROM AUTHOR]

The Omani basement is located spatially distant from the dominantly juvenile Arabian–Nubian Shield (ANS) to its west, and its relationship to the amalgamation of those arc terranes has yet to be properly constrained. The Jebel Ja'alan (NE Oman) basement inlier provides an excellent opportunity to better understand the Neoproterozoic tectonic geography of Oman and its relationship to the ANS. To understand the origin of this basement inlier, we present new radiogenic isotopic data from igneous bodies in Jebel Ja'alan. U–Pb and 40Ar/39Ar geochronological data are used to constrain the timing of magmatism and metamorphism in the jebel. Positive εHf and εNd values indicate a juvenile origin for the igneous lithologies. Phase equilibria modelling is used to constrain the metamorphic conditions recorded by basement. Pressure–temperature (P–T) pseudosections show that basement schists followed a clockwise P–T path, reaching peak metamorphic conditions of c. 650–700°C at 4–7.5 kbar, corresponding to a thermal gradient of c. 90–160°C/kbar. From the calculated thermal gradient, in conjunction with collected trace-element data, we interpret that the Jebel Ja'alan basement formed in an arc environment. Geochronological data indicate that this juvenile arc formed during Tonian time and is older than basement further west in Oman. We argue that the difference in timing is related to westwards arc accretion and migration, which implies that the Omani basement represents its own tectonic domain separate to the ANS and may be the leading edge of the Neoproterozoic accretionary margin of India. [ABSTRACT FROM AUTHOR]

Abstract: A new activity–composition model is presented for green (Mn3+‐absent) yoderite for use in the latest internally consistent thermodynamic data set used by THERMOCALC, for calculations primarily in MgO–Al2O3–SiO2–H2O–O system, where O is a proxy for Fe2O3. P–T grids calculated with our model in the MASH and MASHO system feature invariant points and univariant reaction bundles that are consistent with existing experimental results. Using this new model, we have explored the stability of yoderite in whiteschists, a rare type of high‐pressure rock that conforms closely to the MASHO system. Using a series of calculated models in which composition varies, it is shown that yoderite stability is a function of bulk‐rock SiO2, MgO and Al2O3, where the most important component for stabilizing yoderite is a function of pressure and temperature. The rarity of yoderite in naturally occurring whiteschists is largely related to these compositional factors, with most whiteschists having rock compositions that are too SiO2‐rich and Al2O3‐poor to allow yoderite formation. However, in addition to compositional factors, the calculated P–T stability field of yoderite occurs over thermal gradients that are generally too high to occur in modern‐style subduction zones. As nearly all known whiteschist occurrences are Phanerozoic in age, the near‐complete absence of yoderite in late Neoproterozoic–Phanerozoic whiteschists may be at least partially due to modern subduction systems failing to produce the hotter thermal gradients needed to stabilize yoderite. The provision of this new a–x model for green yoderite allows for more rigorous P–T–X investigations of all whiteschists. [ABSTRACT FROM AUTHOR]

The Windmill Islands region in Wilkes Land, east Antarctica, preserves granulite facies metamorphic mineral assemblages that yield seemingly comparable P-T estimates from conventional thermobarometry and mineral equilibria modelling. This is uncommon in granulite facies terranes, where conventional thermobarometry and phase equilibria modelling generally produce conflicting P-T estimates because peak mineral compositions tend to be modified by retrograde diffusion processes. In situ U-Pb monazite geochronology and calculated metamorphic phase diagrams show that the Windmill Islands experienced two phases of high thermal gradient metamorphism during the Mesoproterozoic. The first phase of metamorphism is recorded by monazite ages in two widely separated samples and occurred at c. 1,305 Ma. This event was regional in extent, involved crustally derived magmatism and reached conditions of ~3.2-5 kbar and 690-770°C corresponding to very high thermal gradients of >150°C/kbar. The elevated thermal regime is interpreted to reflect a period of extension or increased extension in a back-arc setting that existed prior to c. 1,330 Ma. The first metamorphic event was overprinted by granulite facies metamorphism at c. 1,180 Ma that was coeval with the intrusion of charnockite. This event involved peak temperatures of ~840-850°C and pressures of ~4-5 kbar. A phase of granitic magmatism at c. 1,250-1,210 Ma, prior to the intrusion of the charnockite, is interpreted to reflect a phase of compression within an overall back-arc setting. Existing conventional thermobarometry suggests conditions of ~4 kbar and 750°C for M1 and 4-7 kbar and 750-900°C for M2. The apparent similarities between the phase equilibria modelling and existing conventional thermobarometry may suggest either that the terrane cooled relatively quickly, or that the P-T ranges obtained from conventional thermobarometry are sufficiently imprecise that they cover the range of P-T conditions obtained in this study. However, without phase equilibria modelling, the veracity of existing conventional P-T estimates cannot be evaluated. The calculated phase diagrams from this study allow the direct comparison of P-T conditions in the Windmill Islands with phase equilibria models from other regions in the Musgrave-Albany-Fraser-Wilkes Orogen. This shows that the metamorphic evolution of the Wilkes Land region is very similar to that of the eastern Albany-Fraser Orogen and Musgrave Province in Australia, and further demonstrates the remarkable consistency in the timing of metamorphism and the thermal gradients along the ~5,000 km strike length of this system. [ABSTRACT FROM AUTHOR]

Phase equilibria modelling incorporating melt reintegration offers a methodology to create hypothetical rock compositions that may have existed prior to melt loss, allowing the potential prograde evolution of rocks to be explored. However, melt reintegration modelling relies on assumptions concerning the volume of melt that was lost and is generally restricted by the absence of direct constraints on the pre-anatectic mineral assemblages. Mg-rich granulite in the 514-490 Ma Delamerian Orogen in southern Australia contains spinel-cordierite symplectic intergrowths that surround rare, coarse blocky domains of sillimanite. These sillimanite cores, as well as the widespread presence of andalusite in lower grade areas of the southern Delamerian Orogen, suggest that the subsolidus precursor to the granulite contained andalusite. This provides the opportunity to test if melt reintegration modelling of the granulite predicts subsolidus andalusite. Stepwise down-temperature melt reintegration modelling produces a water-saturated solidus after the addition of 12 mol% melt. When modelled at subsolidus conditions, the resulting rock composition produces andalusite-bearing assemblages with andalusite modes similar to the abundance of the sillimanite-cored spinel-cordierite intergrowths. The modelling results from this case study suggest that melt reintegration modelling is a valid method to recreate prograde subsolidus bulk rock compositions. [ABSTRACT FROM AUTHOR]

Metapelitic rocks from the northern Prince Charles Mountains-East Amery Ice Shelf region of the Rayner Complex, East Antarctica, record high-temperature reworking during Cambrian times. Calculated metamorphic phase diagrams for rocks with varying chemical compositions and mineral assemblages suggest that peak temperatures were 800-870°C at pressures of 5·5-6·5 kbar. However, Cambrian-age high-T reworking is patchy and is recorded only in some locations, with other areas recording pristine early Mesoproterozoic-Neoproterozoic assemblages formed during the c. 1000-900Ma Rayner Orogeny. The spatial distribution of reworking may indicate that the comparatively anhydrous residual rock compositions inherited from the Rayner Orogeny were relatively inert to reworking during the Cambrian. Domains that record Cambrian reworking conceivably underwent hydrous retrogression at the end of the Rayner Orogeny and were therefore comparatively reactive during reheating in the Cambrian. High-T reworking during the Cambrian has previously been recognized in the Prydz Bay region at the margin of the Rayner Complex, but not in the northern Prince Charles Mountains. The Eastern Ghats Province in India, which was formerly contiguous with the Rayner Complex, preserves a similarly enigmatic record of Cambrian geochronology, suggesting that the Rayner-Eastern Ghats terrane as a whole may have experienced selective reworking during the Cambrian. The geodynamic setting for the formation of this thermal regime is not well understood, but the attainment of high crustal temperatures may have been facilitated by a reduced capacity for thermal buffering, arising from limited partial melting within a previously dehydrated crustal column. [ABSTRACT FROM AUTHOR]

We present LA–ICP–MS U–Pb monazite and zircon geochronology, trace element chemistry and phase equilibria forward modelling to constrain the P–T–t evolution of the Bunger Hills, East Antarctica. Metasedimentary rocks in the Bunger Hills record evidence for a protracted metamorphic history during the Mesoproterozoic. Taken in isolation, zircon and monazite ages suggest an extremely long duration of high-temperature conditions (ca. 200 Myr). Calculated P–T models indicate metamorphism involved medium pressures of 5.5–7.1 kbar and high to ultrahigh temperatures of 800–960 °C, and that the P–T path likely tracked along a down-pressure to isobaric cooling trajectory. Integrating trace element data from zircon, monazite and garnet indicates that, despite the spread in U–Pb ages, peak metamorphism essentially occurred over the interval ca. 1220–1180 Ma. The age and conditions of Mesoproterozoic metamorphism are consistent with the high-grade metamorphic evolution proposed previously for Stage-2 of the Albany–Fraser Orogeny in southwestern Australia. The P–T–t conditions are interpreted to reflect extension, potentially associated with unloading and exhumation of a collisional orogen following Stage-1 of the Albany–Fraser Orogeny. This is the first study to integrate geochronology, trace element chemistry and P–T modelling to constrain the metamorphic evolution of the Bunger Hills and to interpret these constraints within the context of the now separate terranes of the Musgrave–Albany–Fraser Orogen. The three-way approach adopted in this study demonstrates that zircon and monazite may grow and modify through a number of processes. An integrated petrochronologic approach is therefore essential for investigations on high-grade terranes. [ABSTRACT FROM AUTHOR]