40Ar/39Ar ages from blueschists of the Jambaló region, Central Cordillera of Colombia : implications on the styles of accretion in the Northern Andes

This paper presents the first argon dating of blueschists from the Jambaló area (Cauca Department) in the Central Cordillera of the Colombian Andes. Step-heating 40Ar/39Ar spectra were obtained for mica from several lenses of blueschists including greenschist facies rocks. The blueschists are mainly constituted of preserved lenticular cores in strongly mylonitic rocks, which resulted from retrometamorphic processes that affected the high pressure rocks during their exhumation. The majority of 40Ar/39Ar data points to metamorphic ages close to 63±3Ma, but some ages are older than 71Ma. These Maastritchtian-Danian ages correspond to the timing of exhumation of the blueschists near metamorphic peak conditions, because the dated paragonite and phengite crystallized during development of the mylonitic foliation. The continuous exhumation of this blueschist belt between 71-63Ma reflects the flow on an accretionary system/subduction channel environment that was interrupted by the collision of an intra-oceanic arc with the continental margin. Regional geological correlations suggest that this arc-continent collision also took place in Ecuador. This collisional event, although synchronous with other arc-continent collisions in the Northern Andes, was apparently not related to the formation of the great Caribbean arc, but to an arc built in the southeastern margin of the Caribbean plate

Stable isotope geochemistry and phase-equilibria of coesite-bearing whiteschists, Dora Maira massif, Western Alps

Peak metamorphic temperatures for the coesite-pyrope-bearing whiteschists from the Dora Maira Massif, western Alps were determined with oxygen isotope thermometry. The deltaO-18(SMOW) values of the quartz (after coesite) (delta O-18 = 8.1 to 8.6 parts per thousand, n = 6), phengite (6.2 to 6.4 parts per thousand, n = 3), kyanite (6.1 parts per thousand, n = 2), garnet (5.5 to 5.8 parts per thousand, n = 9), ellenbergerite (6.3 parts per thousand, n = 1) and rutile (3.3. to 3.6 parts per thousand, n = 3) reflect isotopic equilibrium. Temperature estimates based on quartz-garnet-rutile fractionation are 700-750-degrees-C. Minimum pressures are 31-32 kb based on the pressure-sensitive reaction pyrope + coesite = kyanite + enstatite. In order to stabilize pyrope and coesite by the temperature-sensitive dehydration reaction talc + kyanite = pyrope + coesite + H2O, the a(H2O) must be reduced to 0.4-0.75 at 700 750-degrees-C. The reduced a(H2O) cannot be due to dilution by CO2, as pyrope is not stable at X (CO2) > 0.02 (T = 750-degrees-C; P = 30 kb). In the absence of a more exotic fluid diluent (e.g. CH4 or N2), a melt phase is required. Granite solidus temperatures are approximately 680-degrees-C/30 kb at a(H2O) = 1.0 and are calculated to be approximately 70-degrees-C higher at a(H2O) = 0.7, consistent with this hypothesis. Kyanite-jadeite-quartz bands may represent a relict melt phase. Peak P-T-f(H2O) estimates for the whiteschist are 34 +/- 2 kb, 700-750-degrees-C and 0.4-0.75. The oxygen isotope fractionation between quartz (deltaO-18 = 11.6%.) and garnet (deltaO-18 = 8.7 parts per thousand) in the surrounding orthognesiss is identical to that in the coesite-bearing unit, suggesting that the two units shared a common, final metamorphic history. Hydrogen isotope measurements were made on primary talc and phengite (deltaD(smow) = -27 to -32 parts per thousand), on secondary talc and chlorite after pyrope (deltaD = - 39 to - 44 parts per thousand) and on the surrounding biotite (deltaD = -64 parts per thousand) and phengite (deltaD = -44 parts per thousand) gneiss. All phases appear to be in near-equilibrium. The very high deltaD values for the primary hydrous phases is consistent with an initial oceanic-derived/connate fluid source. The fluid source for the retrograde talc + chlorite after pyrope may be fluids evolved locally during retrograde melt crystallization. The similar deltaD, but dissimilar deltaO-18 values of the coesite-bearing whiteschists and hosting orthogneiss suggest that the two were in hydrogen isotope equilibrium, but not oxygen isotope equilibrium. The unusual hydrogen and oxygen isotope compositions of the coesite-bearing unit can be explained as the result of metasomatism from slab-derived fluids at depth

40Ar/39Ar ages from blueschists of the Jambaló region, Central Cordillera of Colombia : implications on the styles of accretion in the Northern Andes

This paper presents the first argon dating of blueschists from the Jambaló area (Cauca Department) in the Central Cordillera of the Colombian Andes. Step-heating 40Ar/39Ar spectra were obtained for mica from several lenses of blueschists including greenschist facies rocks. The blueschists are mainly constituted of preserved lenticular cores in strongly mylonitic rocks, which resulted from retrometamorphic processes that affected the high pressure rocks during their exhumation. The majority of 40Ar/39Ar data points to metamorphic ages close to 63±3Ma, but some ages are older than 71Ma. These Maastritchtian-Danian ages correspond to the timing of exhumation of the blueschists near metamorphic peak conditions, because the dated paragonite and phengite crystallized during development of the mylonitic foliation. The continuous exhumation of this blueschist belt between 71-63Ma reflects the flow on an accretionary system/subduction channel environment that was interrupted by the collision of an intra-oceanic arc with the continental margin. Regional geological correlations suggest that this arc-continent collision also took place in Ecuador. This collisional event, although synchronous with other arc-continent collisions in the Northern Andes, was apparently not related to the formation of the great Caribbean arc, but to an arc built in the southeastern margin of the Caribbean plate

Two stage tectonic history of the SW Amazon craton in the late Mesoproterozoic: identifying a cryptic suture zone

The history of the SW Amazon craton during late Mesoproterozoic times is marked by two separate tectonic events, the first related to collision with southern Laurentia and the second caused by suturing of the Paragua craton. The polycyclic basement rocks of the SW Amazon craton exposed in the Brazilian state of Rondônia were deformed at lower amphibolite conditions during early Grenville times (ca. 1.2–1.15 Ga). This deformation episode is the last of several tectonometamorphic events that affected the granitoid rocks of the Amazon basement throughout the Mesoproterozoic. The southern margin of the Amazon craton during late Mesoproterozoic times is defined by the E–W trending Nova Brasilândia metasedimentary belt, where upper amphibolite to granulite facies rocks from a younger (ca. 1.09 Ga) collisional event are preserved. Temperature–time (T–t) paths for each domain (craton and metasedimentary belt) are constructed using U–Pb, 40Ar/39Ar, and Rb–Sr data for minerals with different blocking temperatures. The T–t paths demonstrate no overlap in the timing or spatial distribution of tectonic and metamorphic activity. The separate cooling histories indicate the presence of a major tectonic boundary between the polycyclic basement rocks and the metasedimentary belt. This structure marks the suturing of the Paragua craton in the late Mesoproterozoic and is evidence that the accretionary history of the present outline of the Amazon craton was completed during the final stages of the amalgamation of Rodinia

57 Fe Mössbauer investigations of manganese-containing spinels

Cation distributions and related order-disorder phenomena in the naturally occurring Mn-spinels: iwakiite, galaxite and franklinite, have been investigated by means of 57 Fe Mössbauer spectroscopy. Previous measurements on an iwakiite sample indicated the existence of Mn-rich and Mn-poor regions. This interpretation has been confirmed in this investigation by further measurements on annealed iwakiite samples. The 57 Fe Mössbauer spectrum of galaxite from Bald Knob, NC, is a well-resolved quadrupole doublet and indicates the presence of a single crystallographic Fe 3+ species. A “galaxite” sample from Thailand exhibited a complex spectrum of Fe 2+ and Fe 3+ quadrupole doublets: This sample has been misidentified and should be regarded as a member of the (Mg, Fe)(Al, Fe) 2 O 4 series. The 57 Fe hyperfine parameters of a rare franklinite from Längban, Sweden, are very close to those for synthetic zinc ferrite, confirming electron microprobe results of an unusually high zinc content.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42930/1/10751_2005_Article_BF02069138.pd