Suites Of Granitic Rocks Research Articles

The NW Amazonian Craton in Guainía and Vaupés departments, Colombia: Transition between orogenic to anorogenic environments during the Paleo-Mesoproterozoic

In order to improve our understanding of the NW-Amazonian Craton evolution, we present new petrographic, geochemical and geochronological analyses of 27 samples from the geotectonic Rio Negro-Juruena Province in eastern Colombia (Guainía and Vaupés departments). New LA-ICP-MS zircon U-Pb ages suggest that the oldest known rocks in Colombia are metamorphic rocks (migmatitic gneisses) with ages between ~ 1850 and ~ 1800 Ma, and gneisses and granitoids with ages between ~ 1800–1720 Ma which form part of the Mitú Complex, interpreted as the result of Statherian collisional and orogenic events (Querari Orogeny). Detrital zircons in low-grade meta-sedimentary sequences of the Tunuí Group (sandstones, conglomeratic sandstones and mudstones), that crop out over almost the entire basement, indicate older than ~ 1770 Ma source rocks. Intrusions of different suites of granitic rocks with syn- to post-collisional affinities suggest a termination of the collisional events between ~ 1600–1500 Ma which had affected the whole region, occasionally metasomatically overprinting parts of the Tunuí meta-sedimentary sequence. The recognizable metamorphic and magmatic-processes finish with ~ 1400–1340 Ma anorogenic granites without signs of tectonic deformation, resembling anorogenic granites in the Western Amazonian Craton in Brazil and the Parguaza Batholith in Venezuela. This study allows us to conclude that the basement records the collision of a continental arc (Rio Negro-Juruena Province) against the NW-Amazonian Craton (Ventuari-Tapajos Province) and its subsequent transition to anorogenic conditions in a continental rift setting long before the actual stable craton conditions.

Comportamento geoquímico de etr durante evolução magmática e alteração hidrotermal de granitos: exemplos da província estanífera de Goiás

The A-type granitic massifs of the Parana (SPP) and Tocantins (SPT) tin sub-provinces have anomalous rare earth element (REE) contents, sometimes 10 3 greater than the chondrite. Two suites of granitic rocks are present in the SPP, g1 and g2. Rocks of g1 suite (g1a, g1b and g1c facies) display an alkalic affinity whereas g2 granites (g2b, g2c and g2d facies) are metaluminous to peraluminous. Granites of the Serra Dourada Massif, in the SPT, are distributed between Border, Main and Porphiritic facies. REE-bearing minerals described in these rocks are apatite, allanite, monazite, basnaesite, REE-oxyfluorides, fluocerite, zircon and xenotime. Those, only thorite wasn’t observed in the Serra Dourada Massif (SPT). The REE are incompatible at the early stages of g1 magma evolution and compatible at the late stages. In all g2 granites, LREE are compatible, whereas HREE concentrations remain unchanged during magma differentiation. In the Serra Dourada granites, the LREE contents are almost constant in all facies while the HREE are incompatible. In all situations, the REE behavior is in agreement with a magma evolution by fractional crystallization. The REE behavior during hydrothermal alteration is contrasting between different granites of both sub-provinces In the SPP massifs, where greisenization dominates, the amount of REE decreases in the altered rocks, whereas in the Serra Dourada Massif, where albitization and biotitization are the main alteration process, the amount of REE increases in the altered rocks. LREE patterns and LREE concentrations of the granites and altered rocks are controlled by allanite, monazite, apatite and the secondary REE minerals. Except for apatite, all these minerals are also important HREE carriers, controlling HREE patterns together with xenotime and thorite. The influence of zircon in these parameters is inexpressive, despite its importance as the main accessory mineral and its enrichment in Y and HREE in some granites.

Granitic Magmatism in the North Qaidam Early Paleozoic Ultrahigh-Pressure Metamorphic Belt, Northwest China

Three suites of granitic rocks are present in the early Paleozoic North Qaidam ultrahigh-pressure (UHP) metamorphic belt, which has eclogite ages of 466-495 Ma. The first suite consists of quartz monzodiorite + granodiorite + monzogranite, the second of monzogranite + two-mica granite + muscovite granite + syenogranite, and the third of granodiorite + monzogranite + biotite granite. The country rocks into which these bodies were intruded consist chiefly of medium- and high-grade Proterozoic gneisses and schists and Paleozoic sedimentary and volcanic rocks. Monzogranite of the first suite has a zircon SHRIMP age of 473 Ma, whereas syenogranite in the second suite has an age of 446 Ma, and granodiorite in the third suite has an age of 397 Ma. Rocks of the first and third suites have similar mineralogies consisting of plagioclase, hornblende, quartz, alkali feldspar, and biotite, whereas those of the second suite consist of potassium feldspar, quartz, muscovite, biotite, and plagioclase. Rocks of the first and third suites have I-type compositions with SiO2 = 61-69 wt%, Na2O/K2O >1, ANK <1, and Eu/Eu* = 0.7-1.0, whereas the second suite consists of S-type granites, with SiO2 = 70-76 wt%, Na2O/K2O <1, ANK >1, and Eu/Eu* = 0.1-0.3. The first suite is interpreted as island arc or active continental margin magmatism, the second as igneous activity related to collision between the Qaidam and Qilian blocks, and the third as post-collisional plutonic activity.

Petrology and Inferred Tectonic Setting of the Mountain Pine Ridge Granitoids, Maya Mountains, Belize

The Mountain Pine Ridge batholith is the largest of three major granitoid intrusions that crop out in the Maya Mountains of Belize. These plutons intrude a thick sequence of Upper Paleozoic metasedimentary and metavolcanic rocks known as the Santa Rosa Group. The mineralogy of the granitic rocks at Mountain Pine Ridge varies and the rocks are classified into four petrographic types—biotite leucogranite, muscovite leucogranite, granodiorite, and tonalite. The essential felsic minerals in these rocks include quartz, microcline, plagioclase feldspar ± perthite, and muscovite, whereas the only mafic mineral is biotite. Microprobe analysis shows that significant differences exist between the biotites of the different petrographic types, especially in the levels of Al(vi), Mn, Mg, total Fe, and the Fe/Fe+Mg ratio. Major, minor and trace-element chemistry of the four petrographic types indicates that the Mountain Pine Ridge batholith is composed of three different suites of granitic rocks. Although all three sui...

Rb–Sr dating of Paleozoic(?), Mesozoic, and Cenozoic intrusive rocks, Okanagan Lake region, southern British Columbia, Canada

Sr isochron plots distinguish three different suites of granitic rocks in the vicinity of Okanagan Lake. Voluminous granites sampled over a large area east and west of the lake have scattered 87Sr/86Sr isotopic ratios (0.706–0.710) that indicate either a Paleozoic age (possibly as old as Devonian) or heterogeneous and high initial ratios in Mesozoic granites. The second suite, a composite from localities near Whiterocks Mountain, gives an isochron date of 150 ± 5 Ma with an initial 87Sr/86Sr ratio of 0.7039 ± 0.00015. The third suite, from the Whiteman Creek stock and granites along the shores of Okanagan Lake and Wood Lake, gives an isochron date of 59 ± 2 Ma with initial ratio 0.70515 ± 0.00015. The secular increase in initial ratios reflects crustal thickening and evolution. These new dates are consistent with Paleozoic (circa 400 Ma), Mesozoic (180 and 150–160 Ma), and Cenozoic (50 Ma) igneous episodes previously recognized in the border zone between the Intermontane and Omineca belts in southern British Columbia.

Some problems of interpreting the correlations between the modal variables in granitic rocks

Correlation coefficients of modal variables from several suites of granitic rocks have been calculated and tested for significance using the Chayes-Kruskal and Chayes methods. The results show that although Chayes' remaining-space transformation does, in general, weaken the original proportion correlations, the positive and low-negative original proportion correlations tend to increase in absolute value because of a tendency of positive increment of the covariance by such transformation. However, no satisfactory procedure for choosing the variable to be transformed (V2)could be determined, and testing of the significance of remaining-space correlations is found to be highly problematic. It is shown also that the Vistelius-Sarmanov procedure of calculation of correlation coefficients from closed-table data does not effectively eliminate the closure effect. It is concluded that assigning statistical significance to the correlation coefficients between modal variables is, in general, unsatisfactory, except where the number of variables ⩾8,the sample size is large (say, ⩾30),and there is no negative element in the variance vector of the corresponding open variables.