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V-Pb and K-Ar AGES of PICHI MAHUIDA GROUP, CRYSTALLINE

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V-Pb and K-Ar AGES of PICHI MAHUIDA GROUP, CRYSTALLINE ·.... -~' - -r-- "'-· ·· _.. -----r - --_ . _ ~ DOC. 427 . r-:"" , - , .~ V-Pb AND K-Ar AGES OF PICHI MAHUIDA GROUP, CRYSTALLINE BASEMENT OF SOUTH-EASTERN LA PAMPA PROVINCE, ARGENTINA ~ / Hugo Tickyj' , Miguel Angelo Stipp Basei", Ana Maria Sato'' and Eduardo Jorge Llambias3 1 CONICET - Universidad Nacional de La Pampa. Av. Uruguay 151,6300 Santa Rosa, La Pampa, Argentina. [email protected] . 2 Centro de Pesquisas Geocronol6gicas, Universidade de Sao Paulo. Rua do Lago 562, Sao Paulo, Brasil. [email protected] 3 Centro de Investigaciones Geol6gicas, CONICET-Vniversidad Nacional de La Plata. Calle 1 N° 644. 1900 La Plata, Argentina. [email protected]; [email protected] Keywords: U-Pb, granitoids; Famatinian orogenic cycle, La Pampa province, Argentina INTRODUCTION GEOLOGY The crystalline basement of south-eastern La The Pichi Mahuida Group is a post-orogenic Pampa province consists of Upper Cambrian (?) ­ unit composed of porpliyritic muscovite-biotite Ordovician, very low to medium-grade metamorphic granites and granodiorites. · It has a minimal rocks (Las Piedras Metamorphic Complex) and Upper deformation and in this aspect it is diferent from Cambrian - Lower Devonian, non-deformed, post­ deformed granites ofCerro de Los Viejos Complex . orogenic granitoids (Pichi Mahuida Group), which are The main outcrops are located at the riversides considered to belong to the Famatinian orogenic. of Curac6 and Colorado rivers near the locality of cycle. In addition to the former rocks, granitic Pichi Mahuida (Fig. 1). In this area four plutonic units orthogneisses metamorphosed during the Late were identified: 1) Rio Colorado Granodiorite, 2) Paleozoic times (Cerro de Los Viejos Complex) SaIto Andersen Granodiorite, 3) Curaco Granite and represent the Gondwanic orogenic cycle (Tickyj, 4) Puesto Sainges Granodiorite-Granite. 1999; Tickyj et al., 1999). The relationship with the country rocks are In this contribution we present new V-Pb and poorly known due to .the small and discontinuous K-Ar ages obtained for Rio Colorado Granodiorite outcrop situation. The best exposures are in Salto and SaIto Andersen Granodiorite that belong to Pichi Andersen, where a granodiorite intrudes Mahuida Group. These granitoids are located at metasedimentary rocks that belong to Las Piedras 38°52'S-65°00'W on the border area between La Complex. Pampa and Rio Negro provinces, Republica All studied units are characterized by Argentina. The sample preparation, chemical analyses subalkaline, potassic and peraluminous features. The and mass spectrometry for U-Pb and K-Ar methods granodiorites display a calcalkaline trend, although it were carried . out at Centro de Pesquisas is not possible to extend this trend to the granites due Geocronologicas, Sao Paulo (Brasil). to compositional gap between them. Moreover, the Previous ' geochronological data in this area obtained ages suggest t hat granites and granodiorites comprise aRb-Sr date of 500 ± 35 Ma (whole rock, could have independent evolutions. assumed Ri= 0.707) reported by Halpern et ai. (1970) The Rio Colorado pluton is of granodioritic and three Rb-Sr isochrones of 530 ± 30 Ma, 430 ± 15 composi tion . It has a porphyritic texture, with K­ Ma and 365 ± 15 Ma (whole rock, assumed Ri= feldspar megacrysts up to II em long oriented by ·0.707), obtained by Linares et ai. (l980).Besides, a magmatic flow . Zoned plagioclase, quartz, K­ . detailed geological study on these granitoids was feldspar, biotite and muscovite are the main minerals. performed by Sato et ai. (1996) who obtained for Micaceous enclaves are abundant, while mafic Curaco Granite a Rb-Sr age of 397 ± 25 Ma (whole microgranular enclaves are scarce. This unit show a rock, calculated Ri= 0.724). Si02 range of 64-68%, a weak peraluminous character 139_ \ • .....,r, . ... ,; - Actus II South American Symposium on Isotope Geology eUTaW --------i.lli..r-- ___ ./ / Dykes ~ Gr~!~hillcd border o I 2 Hm • Granodiorite I e=--t I rmJ Metamorphic rocks Figure 1. Granitic outcrops ofPichi Mahuida Group at the border area between La Pampa and Rio Negro provinces. (A/CNK= 1.04-1.19) and high K20INa20 ratios (1.1­ In Puesto Sainges a small outcrop of 1.4). metasedimentary rocks was intruded by two granitic In the area of Salta Andersen Dam, bodies. One of them is a biotitic granodiorite, which metasedimentary rocks were intruded by a gray has inequigranular texture with plagioclase granodiorite. The contact zone displ ay a metamorphic phenocrysts up to 1.5 em long. The mineral paragenesis with andalusite and cordierite. The pluton composition is very similar to Salta Andersen encloses xenoliths of the country rocks and contains Granodiorite, although without amphibole. Mafic abundant joints filled with quartz or calcite. The microgranular enclaves are common. The other pluton texture is inequigranular, with few plagioclase is a two mica granite, which intruded both the phenocrysts up to 1. em long. Zoned plagioclase, granodiorite and metasediments. Large xenoliths of quartz, K-feldspar, biotite and amphibole are the main them are enclosed by the granite. minerals. Mafic microgranular enclaves are common , while those of micaceous type are scarce. The K-Ar and V-Ph AGES OF RIO COLORADO granodiorite has a Si02 content of 65-67%, with a GRANODIORITE K20IN a20 ratio of 1.2-1.3 and a weak peraluminous character (NCNK=I.09-1.17) (Sato et al. , 1996). Separation of biotite and zircon on a sample of Th e Curaco pluton is a two mica S-type Rio Colorado Granodiorite was performed for K-Ar leucogranite intrusive into Rio Colorado dating and V-Pb dating. Granodiorite. It has a porphyritic texture, with K­ The K-Ar analysis on biotite gave an age 437 ± feldspar phenocrysts up to 3 em long and contains 14.1 Ma (Fig. 2). small micaceous enclaves « 1 ern). It was intruded by Conventional V-Pb analysis were carried out on abund ant aplitic and pegmatitic dikes related to the four fractions of zircons (Figs . 3 and 4a). The final stage of the magm atic evolution. This pluton has discordia line obtained (MSWD=70) has an upper a Si02 range of 73-75%, a peraluminous character intercept with the Concordia at 500 ± 27 Ma. This (AICNK=1.16-1.22) and high K 20/Na20 ratio (1.3­ result represents the age of crystallization of the 1.6) (Sato e l al., 1996). granodiorite. -e-" Sample Location Rock Analized %K Error (%) Ar40 rad *IO-6 Ar40 atm Age (Ma) material (ccSTP/g) (%) GCl-1 Rio Curac6 Granodiorite Biotite 5.7771 1.8551 110.89 13.25 437.0 ± 14.1 F igure 2. K-Ar analytical data. Fraction (I) Weie.ht U (oom)1 Pb (ppm) 206Pbp04Pb 206Pb/238 U 207Pb/23SU 207PbP06pb Azes (5) (11:) (2) (2) Observed (3) (4) (4) (4) 2OIpbfl8u\ 2lJipty2l\ 207pbf!lSpb GeZ 01 Rio Colorado Granodiorite SPU64 M(-2)AA 0.00066 1332.6 99.145 1381.215 0.072362 0.576435 0.057774 460 462 521 SPU65 Mn)AA 0.00051 425.7 29,98 788.2084 0.066441 0.521389 0.056915 415 426 488 SPU66 M(-I) 0.00043 457.84 35.53 1590.837 0.075965 0.601495 0.057427 472 478 508 SPU67 NM(-2) 0.00012 599.0 55.57 547.3753 0.085998 0.674335 0.056870 532 523 486 SAZ 0 SaIto Andersen Granodiorite SPU251 M(-6) 0.00085 110.2 8,99 542.4758 0.072174 0.60659 0.060955 449 481 638 SPU253 M(-4) 0.00090 188.0 14.27 1024.8 0.070173 0.57059 0.058973 437 458 566 SPU254 M(-3) 0.00110 223.2 18.84 344.887 0.069769 0.54314 0.056461 435 440 470 SPU293 M(-5) 0.00032 213.3 18.69 357.7946 0.073992 0..663601 0.065046 460 517 776 SPU294 M (-6) 0.00036 4.5 3.5 41.82 258.0645 0.072075 0.59332 0.059704 449 473 593 1: Non magnetic fractions (Frantz separator at 1.5 amp. current); 2: Total U and Pb concentrations corrected for analytical blank; 3: Not corrected for blank or non-radiogenic Pb ; 4: Radiogenic Pb corrected for blank and initial Pb ; U corrected for blank; 5: Ag es given in Ma using Ludwig Isoplot Program (1993), decay constants recomm ended by Steiger and Jager (1977). Figura 3. V-Pb analytical data. 0.092 , 0.076 ,..---------,----------, ..3~ 2llllPbf3"u SAZO ". ." 0.088 NM('2~"" Saito Anderse/n .: Gez - 01 Granodiorite " ." 530 0.084 Rio Colorado 51 ... 0.074 .......'" M(·5) Granodiorite .... O.OBO / ' w<·i{)~;:;) 0.076 47 .7M(_1) 0.072 45 ./ . Upper Intercept at: 0.072 ", Intercepts at: ......M(-2)AA 500 ± 27 Ma 440 ." ...... M(-4) Lower - 431 ± 12 Ma 0.06B MSWD =70 0.070 ...... Upper - 2580 Me (FORCED THRU ORIGEN) a) ,'" 1 MSWD=3.6 0.064 b) 0.060 +-~-+----t---__il--~--;--~__I_-~_l 0.068 -l<---1l:.-...-+--~__i-~--+--~__I_-~--l 0.48 0.52 0.56 0.60 0.64 0.68 0.72 0.50 0.54 0.58 0.62 0.66 0.70 . F igure 4. Concordia diagrams: a) Rio Colorado Granodiorite, b) Saito Andersen Granodiorit e. 141 <P- Aetas II South American Symposium on IsotopeGeology The value obtained by the K-Ar method on REFERENCES biotite is probably related to thermal effects caused by later magmatic or metamorphic activities. - Halpern, M.E., Umpierre, M. and Linares, E., 1970. Radiometric ages of crystalline rocks from south U-Pb AGE OF SALTO ANDERSEN America as related to Gondwana and Andean GRANODIORITE Geologic Province. Conf. Problemas Tierra S6lida. II: 345-356, Buenos Aires. For this unit five fractions ofzircons were obtained to Linares, E., Llambias, E.!. and Latorre, C.O., 1980. perform a conventional U-Pb dating (Figs 3 and 4b).
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