In‐situ monazite dating of a microdiamond‐bearing gneiss, Fjortoft,  Norway: Age pattern in relation to garnet zoning and growth history

Garnet-kyanite gneiss at Fjørtoft (FGKG), Norway forms a distinctive unit in the Western Gneiss Region attributed to the Blahø Nappe of the Caledonides (Dobrzhinetskaya et. al., 1995; Robinson, 1995). It lies against orthogneiss enclosing mantlederived peridotites. Both units give evidence of diamond-grade metamorphism, which calls into question their allochthonous relationship. We have conducted a detailed in-situ microprobe monazite dating programme of this important but enigmatic lithology in order to elucidate the age provenance of the FGKG and its history of garnet growth and partial melting during subduction and exhumation. The FGKG is a metapelitic blastomylonite but low-strain enclaves indicate a prior migmatitic history. Garnets megacrysts are zoned with inclusions of kyanite and low-Si phengites in the cores. Overgrowths enriched in Ca and Mg enclose inclusions of perthite, quartz, biotite, kyanite and graphite. No evidence for high-P phases such as has been found in-situ. Core growth is estimated at 0.8GPa, ~550oC and overgrowths at ~1.1GPa, ~800oC. Evidence for UHP conditions (875oC at 4GPa) from nearby eclogites and peridotites requires a peak UHP paragenesis of grt+phe+jd+ky+coes+di for the FGKG. This would have undergone decompression melting by decomposition of phengite. We infer that the UHP assemblage along with almost all diamond has been obliterated during exhumation. Monazite inclusions are abundant throughout the garnet megacrysts. EMP analysis of inclusions gives an age pattern that correlates with garnet zoning. Monazites in garnet cores predominantly give ages of ~1080Ma (i.e. Sveconorwegian orogeny), while those in overgrowths give a bimodal distribution with another age peak around 425Ma (Scandian orogeny). Monazites in garnet overgrowths often have two age domains with either the Sveconorwegian or Scandian ages, suggesting re-setting or growth of new monazite. We explore three scenarios: (1) monazites were detrital and reset during entrapment by garnet during Scandian UHPM and decompression melting; (2) monazites record Sveconorwegian garnet growth, then diffusional readjustment of both phases during Scandian decompression melting; (3) monazite grew with 9th International Eclogite Conference 2011, Mariánské Lázně, Czech Republic 14 Sveconorwegian garnet and again during post-UHP Scandian decompression melting with further garnet growth. Scenarios (2) and (3) require destruction of UHP garnet during exhumation. The lack of Sveconorwegian ages in the orthogneisses suggests that they lacked a common metamorphic history with the FGKG until the Scandian orogeny. In case (1) the FGKG was a post-Sveconorwegian sedimentary cover to an orthogneiss basement. In cases (2) & (3) it underwent post-Sveconorwegian tectonic emplacement onto Baltica basement before they were subducted together during the Scandian. Dobrzhinetskaya, L.F., Eide, E.A., Larsen, R.B., Sturt, B.A., Trønnes, R.G., Smith, D.C., Taylorm W.R. & Posukhova T.V., 1995. Microdiamond in high-grade metamorphic rocks of the Western Gneiss region, Norway. Geology, 23; no. 7; p. 597-600. Robinson, P., 1995. Extension of Trollheimen tectonostratigraphic sequence in deep synclines near Molde and Brattvåg, Western Gneiss Region, southern Norway, Nor. Geol. Tidsskr., 75, 181 – 198.