Metamorphic samples from the South American Andes

Ben Goscombe, March 2014. Unpublished ITAR Report.

[1] Cordillera Blanca, Peru

The Cordillera Blanca in Peru contains granites of probable Cenozoic age. These granites have contact metamorphic aureoles with new ITAR peak metamorphic PT determinations of 535±6 ºC, 3.6±0.4 kb and 42.5±4.2 ºC/km. The petrology in these aureoles indicates anticlockwise P-T paths with prograde isothermal loading.

Sample P1:

Sample of contact metamorphic pelite in a medium-grained hornblende leucogranite aureole, collected from Wilkowain village in the Cordillera Blanca, North Peru. The metapelite is dark fine-grained graphite-muscovite-andalusite-garnet-biotite schist. The fine-grained schistose matrix contains strained single quartz grain ribbons and an assemblage of graphite, quartz, orange siderophyllitic biotite, fine ilmenite, muscovite, oligoclase plagioclase and rare ripidolite chlorite and apatite. Early formed andalusite occurs as porphyroblasts that are enveloped and boudinaged by the foliation. Biotite and chlorite growth occurred in the inter-neck zone of andalusite boudins and in pressure shadows. Garnet occurs as small post-kinematic, idioblastic porphyroblasts without inclusions growing across the foliation. Garnet is Mg-spessartine-almandine with Mn-Mg-almandine rims. Compositional maps and quantitative analyses of garnet show typical zoning of; increasing Fe and Mg, decreasing Mn and Ca.

Sequence of mineral growth = and porph > qtz-bt-ms-chl-gr-pl-ilm-ap foli > grt porphs

Prograde T, P and T/depth conditions from garnet cores are 513±6 ºC, 2.3±0.4 kb and 63.7±10 ºC/km, defined by Gaidies et al. (2008) pseudosection, with Fe, Mg, Mn, Ca isopleths intersecting well. THERMOCALC average PT determinations from mineral cores give similar result, with sigfit of 0.99 (<1.39 for 95% confidence). 4 PT determinations reduce errors. The Si in white mica geobarometer gives similar P of 2.6 kb. Peak metamorphic conditions are based on THERMOCALC v3.25 average PT calculations (Powell and Holland, 1988). Peak conditions from matrix rims are 535±6 ºC, 3.6±0.4 kb and 42.5±4 ºC/km, with high calculation confidence of 12. PT calculations assume XH2O of 1.0. The evolving mineral parageneses determined from petrology such as andalusite over-grown by garnet-biotite assemblages, in conjunction with the PT determinations, constrain a loading-heating prograde P-T path at low pressures, with probable anticlockwise loop.

[2] Pre-Cordillera Terranes, Patagonia

Pre-Cordillera Terranes in Patagonia experienced Cretaceous orogenesis. A single U-Pb zircon metamorphic age determinations from the Cordillera Darwin is 72.6±1.1 Ma (Maloney et al., 2011). PT determinations in literature from the Cordillera Darwin give peak metamorphic conditions of 620 ºC, 12.0 kb and 15.0 ºC/km (n=1), with clockwise P-T paths and isothermal decompression (Maloney et al., 2011). New ITAR peak metamorphic conditions from the Baraloche region in north Patagonia have a low-P sub-domain of 562±35 ºC, 3.2±0.5 kb and 50.2±8.4 ºC/km and high-P sub-domain of 600±38 ºC, 7.2±0.7 kb and 23.8±2.8 ºC/km. P-T evolutions are clockwise paths with prograde isothermal loading.

Sample Pat2:

Sample of granodiorite from pre-Cordillera Terrane on the NW side of Mt Fitzroy, Patagonia. Medium-grained hornblende-biotite granodiorite with igneous texture of very coarse inter-locking stubby plagioclase laths. Matrix assemblage consists of plagioclase, quartz, brown biotite and brown igneous edenitic hornblende. Hornblendes have green to green-blue rims that are probably metamorphic. Plagioclase laths are strongly zoned and biotite is uncommonly retrograded to chlorite. Magmatic crystallization conditions are based on Al in hornblende geobarometer. 6 determinations reduce errors. Crystallization conditions are 5.0±0.5 kb and a crystallization temperature of ~750 ºC is assumed.

Sample Pat4:

Sample from a pre-Cordillera terrane at Baraloche Pass in north Patagonia, Argentina. Fine grained metapelite schist with quartz, muscovite, biotite and late-stage porphyroblasts of andalusite across the main foliation. Rock texture is medium grained, strongly schistose with contorted foliation and bent micas. Foliation assemblage consists of polygonal granoblastic quartz ribbons, oligoclase plagioclase, graphite, Mn-ilmenite, Mg-muscovite and green-brown siderophyllitic biotite. Andalusite occur as late porphyroblasts across the foliation. Garnet occurs as pseudomorphs replaced by green-brown biotite and minor muscovite and chlorite and formed during flattening. Garnet also occurs as relict grains within pseudomorphs. The original garnet appears to be mostly pre-kinematic, being mostly enveloped and only rarely abutting the main foliation. Garnet is Ca-Mn-Mg-almandine with Ca-Mg-almandine rims. Compositional maps and quantitative analyses of garnet shows typical zoning with diffusion rims of; increasing Fe with thin low rims, increasing Mg, decreasing Mn with thin high rims and decreasing Ca with thin low rims.

Sequence of mineral growth = grt pseudo > qtz-pl-bt-ms-gr-ilm-mnz foli > and porphs > bt-ms-chl

Prograde T, P and T/depth conditions from garnet cores are 520 ºC, 8.8 kb and 16.9 ºC/km, defined by Gaidies et al. (2008) pseudosection, with Fe, Mg, Mn, Ca isopleths intersecting well. This high-P result may not be consistent with the later andalusite-bearing parageneses. Peak conditions are based on pooled average T and average P reactions calculated in THERMOCALC v3.25. Peak conditions from garnet rims and matrix cores are 562±35 ºC, 3.2±0.5 kb and 50.2±8 ºC/km, with a moderate calculation confidence of 9. PT calculations assume XH2O of 1.0. The evolving mineral parageneses determined from petrology such as garnet over-grown by andalusite, in conjunction with the PT determinations, give a poorly constrained probable isothermal decompression and clockwise P-T path for this sample.

Sample Pat7:

Sample from pre-Cordillera Terrane at Baraloche Pass in north Patagonia, Argentina. Coarse-grained hornblende, plagioclase amphibolite with large (1.5-2.0 cm) garnet. Possible xenolith in wide granite dykes. Garnet porphyroblasts are in a coarse-grained polygonal granoblastic matrix consisting of dominant green-blue ferroan pargasitic hornblende and minor andesine plagioclase, rutile, apatite and Mn-ilmenite. Plagioclase is partially sericitized. Garnet is Ca-Mn-pyrope-almandine and contains ilmenite inclusions and is pulled apart and in filled and retrograded by pycnochlorite and magnetite. There are no moats around the garnets. Quartz has not been identified by electron microprobe. Quantitative garnet analyses indicate atypical growth zoning of; decreasing Fe with thin high rims, increasing Mg with thin low rims, flat Mn with thin high rims and flat Ca with thin low rims.

Sequence of mineral growth = ilm incs > pl-hbl-ilm-ap-rt matrix + grt porph > chl-mag retro > seri

Peak metamorphic conditions are based on the intersection of average PT-aH2O array and fluid independent average PT calculated in THERMOCALC v3.25. Peak metamorphic conditions from matrix rims are 600±38 ºC, 7.2±0.7 kb and 23.8±3 ºC/km, with high calculation confidence of 11. Intersection of average PT-aH2O array and fluid independent average PT give a calculated aH2O of 0.3. The evolving mineral parageneses determined from petrology such as garnet over-grown by andalusite, in conjunction with the PT determinations, constrain a probable isothermal decompression and clockwise P-T path for this sample.