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This study investigates distinctive oriented, needle-shaped multiphase inclusions found within cores of garnets from felsic granulites of the Upper Member of the Bigelow Brook Formation and the Brimfield Schist (northeastern Connecticut, U.S.A.). The rocks crop out in the southern end of the Central Maine Terrane and thus are a part of the Acadian/Neoacadian Orogen. The typical mineral assemblage is garnet + sillimanite + K-feldspar + plagioclase + quartz + cordierite + biotite ± spinel. The sillimanite is commonly pseudomorphous after kyanite.The multiphase needle inclusions and associated exsolved needles of rutile and ilmenite are mostly oriented parallel to <111> of garnet. The multiphase needles contain various combinations of quartz, micas, chlorite, rutile, graphite, a siliceous compositionally variable phase ("Phase-CV"), Zn-spinel, apatite, zircon, and rare ilmenite. We hypothesize that they represent inclusions of fluid that adhered to exsolving Ti±Fe oxide needles (mostly rutile) or extended along zones of weakness in garnet. This requires that multiphase needle formation occurred in response to cooling and/or decompression. The needles ultimately decrepitated during retrogression. We note that micaceous needle-shaped multiphase inclusions are rarely described; the closest analogs of which we are aware are found in UHP garnets of the Erzgebirge (Perchuk 2008).The Brimfield Schist in this area underwent ultrahigh-temperature metamorphism (UHTM) of ~1000 °C at a minimum pressure of ~1 GPa (Ague et al. 2013). Here we provide new temperature estimates for the adjacent Upper Member of the Bigelow Brook Formation. Ternary feldspar reintegration using the activity model of Benisek et al. (2004) and Zr-in-rutile thermometry (Tomkins et al. 2007) give average temperatures of ~990 and ~1010 °C, respectively, at 1 GPa for this unit. Therefore, the recently discovered UHT zone in the Brimfield Schist of northeastern Connecticut extends to the east to include the Upper Member of the Bigelow Brook Formation. Consequently, the first confirmed regional UHT locality in the United States is larger than initially recognized, and is at least 25 km long and 5-10 km wide. The oriented, elongate multiphase inclusions and petrographically obvious oriented Ti±Fe oxide needles may be useful indicators of extreme temperature and/or pressure rocks in other field areas.

This study describes halos of rutile AE apatite needles and/or plates centred on quartz (inferred former coesite) inclusions or multiphase inclusions in garnet. Both types of central inclusions are surrounded by cracks. The multiphase inclusions contain mica or carbonate minerals and are interpreted to represent decrepitated fluid inclusions. Examples from two localities are examined: (i) ultrahigh-temperature (UHT) metapelitic gneisses from the southern end of the Central Maine Terrane in northeastern Connecticut, USA (rutile only) and (ii) ultrahigh-pressure (UHP) diamondiferous saidenbachite from the Saxonian Erzgebirge (rutile and apatite). The needles of apatite or rutile are typically oriented in three directions within garnet. Chemical zonation in garnet shows clear Ti or P depletion halos corresponding spatially with the rutile or apatite inclusion halos. The radii of the inclusion halos, when measured from the centre of the central inclusions, are about two to several times the radii of the central inclusions. We propose that the inclusion halos of rutile AE apatite formed by exsolution out of Ti-bearing and/or P-bearing garnet during retrogression. Because of its strength, garnet can internally maintain higher pressures than the surrounding matrix. Nonetheless, if the inclusion pressure is significantly greater than the confining lithostatic pressure, then deformation of the host garnet can occur. During exhumation, high internal pressures relative to the matrix can result from retention of fluid entrapment pressure or a phase change with a large positive volume increase (e.g. coesite ? quartz). The differential stress imposed on the garnet adjacent to the central inclusion preceding and during mechanical failure would create dislocations and other crystalline defects which are ideal sites for exsolved precipitates to nucleate and grow. The strain-induced exsolution hypothesis is consistent with observations. (i) The radii of the halos are roughly consistent with the pressure vessel model or the multi-anvil model for an inclusion, which predict that differential stress will drop off sharply away from the boundary of the central inclusion into the host garnet. (ii) The rutile or apatite inclusions formed in Ti-or P-bearing garnet adjacent to ruptured central inclusions; rutile or apatite are rare or absent elsewhere in garnet, even in areas of high-Ti or high-P concentration. In addition, rutile is absent in areas surrounding the central inclusion that were depleted in Ti prior to rupturing (e.g. garnet rims that lost Ti during retrogression). Thus, both elevated Ti AE P concentrations and proximity to inclusions which deformed the host garnet were necessary for halo formation. (iii) Reintegrated garnet Ti or P contents in and adjacent to the halos obtained using wide-beam electron probe microanalysis are consistent with local derivation of the Ti or P necessary to form rutile and apatite from garnet. Elevated Ti AE P concentrations in aluminous garnet are mostly found in UHT, HP/UHP, granulite and mantl...

Inclusions of relic high pressure melts provide crucial information on the fate of crustal rocks in the deep roots of orogens during collision and crustal thickening, including at extreme temperature conditions exceeding 1000°C. However, discoveries of high pressure melt inclusions are still a relative rarity among case studies of inclusions in metamorphic minerals. Here we present the results of experimental and microchemical investigations of nanogranitoids in garnets from the felsic granulites of the Central Maine Terrane (Connecticut, US). Their successful experimental re-homogenization at ~2 GPa confirms that they originally were trapped portions of deep melts and makes them the first direct evidence of high pressure during peak metamorphism This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America. The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.

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