In-situ garnet Lu-Hf geochronology from the contentious Belomorian high-pressure rocks.

Eclogites are a characteristic product of subduction-driven metamorphism and generally regarded as mineralogical evidence for modern-style plate tectonics. Consequently considerable effort has been expended to determine when eclogite appeared in the geological record, including claims of Archaean-aged eclogite.  One area of such contention is the Belomorian Orogenic Belt in NW Russia.  There are two schools of thought regarding Belomorian high pressure metamorphism.  One is that metamorphism occurred at c. 2.7 Ga, representing potentially the oldest occurrence of eclogite (eg Volodichev et al 2021; Minerals, 11, 1029).  The other view is that high pressure metamorphism occurred at around 2 Ga, forming part of the global set of c. 2.Ga eclogites (eg Yu et al., 2017; Journal of Metamorphic Geology, 35, 855-869). 

A combination of sample-scale X-ray mapping, laser ablation ICPMS and Time-of-Flight mapping has been used to delineate two generations of garnet in a sample from the well-studied Stolbikha Island locality in the Belomorian Belt.  The sample contains voluminous diopside-plagioclase symplectites whose re-integrated composition is consistent with the former presence of pyroxene with X_jd ~ 0.26-0.3. The symplectites overprint hornblende that probably formed during melt crystallisation.  The cores of the older generation garnet are comparatively HREE rich, and give an in-situ laser ablation Lu-Hf age of 2490 ± 40 Ma.  The second generation garnet is comparatively HREE depleted, and gives a Lu-Hf age of 1875 ± 86 Ma.  The comparatively large uncertainties reflect low Lu concentrations in the analysed garnets.  Both generations of garnet preserve partially relaxed major element zoning, and given the apparent diffusivities of major elements relative to Lu and Hf, it is probable both derived ages are meaningful.  An important difference between the two garnet generations is the presence of rutile inclusions in younger garnet and their absence in older garnet.   P-T modelling suggests c. 2.5Ga metamorphism occurred at c. 6-8kbar at temperatures > 750°C. For the younger mineral assemblage, P-T modelling taking into account the presence of the older garnet as well as the inferred clinopyroxene composition, combined with Zr-in rutile thermometry from rutile-qtz-zircon clusters in c. 1875 Ma garnet, gives c. 1.5 GPa and 700°C.   

The results indicate Belomorian Orogen high pressure metamorphism occurred at c.1875 Ma, confirming the conclusions of other workers (eg Yu et al 2017) that high pressure metamorphism is Palaeoproterozoic in age.  However we find no evidence for notably elevated pressures (> 2GPa) for the metamorphism at c. 1875 Ma as suggested by some previous workers, or evidence for high pressure metamorphism at c. 2.7 Ga. The Belomorian high-P rocks belong to the now globally distributed suite that records the emergence of eclogite in the initial stages of Nuna assembly. The general emergence of eclogite in the geological record at around 2 Ga probably reflects a combination of subduction into a cooler mantle and continental processes that facilitated preservation.