Petrogenesis of the postcollisional Middle Devonian monzonitic to granitic magmatism of the Sierra de San Luis, Argentina

WEMMER, KLAUS; LÓPEZ DE LUCHI, MÓNICA G.; NOLTE, NICOLE; SIEGESMUND, SIEGFRIED

LITHOS

ELSEVIER SCIENCE BV

Año: 2017

0024-4937

AbstractMiddle Devonian granitoids intruded theEastern Sierras Pampeanas basement ca. 600 km east of the inferredproto-Pacific margin of Gondwana along which a ca. 390 Ma collisional event developed.In the Sierra de San Luis, voluminous Middle Devonian (393-382 Ma) batholithsare composed of I-to A-type hybrid Monzonite and Granite suites. Shoshonite andsubordinated high-K series, stocks, synplutonic dikes and enclaves  make up the Monzonite Suite; rocks aremetaluminous alkali-calcic magnesian porphyritic or equigranular monzonite,quartz monzonite, monzodiorite and scarce monzogabbro. High-K and subordinatedshoshonite series metaluminous to mildly peraluminous magnesian alkali-calcicto calc-alkalic porphyritic or equigranular quartz monzonite, granodiorite, monzograniteand equigranular leucomonzogranites make up the Granite Suite plutons and  batholiths. Only a small group of highlyevolved granites are ferroan. SiO2 (46?62%), Cr, Ni, V, Sc, LILE,LREE, Th, Zr and variable, Sr/Y, (La/Yb)N and (Tb/Yb)N, smoothEu/Eu*, moderate Na2O (ca 3.5), and troughs at Nb and Ta forMonzonite Suite rocks suggest an subduction-related enriched lithosphericmantle source.  Sm-Nd data (TDM0.98 -1.08 Ga, εNd(380Ma) 0.66-1.47) and 87Sr/86Sri(0.703520-0.704203) are compatible with an enriched mantle source. Themetaluminous porphyritic quartz monzonite-monzogranite and the mildlyperaluminous equigranular biotite monzogranites of the Granite Suite arecharacterized by relatively moderate Al2O3, CaO, and 87Sr/86Sri,high LILE, Cr,  variable Sr/Y, (La/Yb)Nand Eu/Eu* and low Rb/Sr (<1.2) suggest a mafic source. The porphyritic monzogranite(TDM 1.20-1.28 Ga , εNd(380Ma) -3.02 to -3.3, 87Sr/86Sri0.706578?0.707027) and the biotite monzogranites (TDM 1.31 Ga, εNd(380Ma)-3.3, 87Sr/86Sri 0.707782) would share acommon source. The equigranular alkali-calcic leucomonzogranites are characterizedby Rb/Sr >1.5, ASI 1.05-1.18, and Ga/Al 2.6-3.9, εNd(380Ma) -3.74to-3.95 and (87Sr/86Sr)i 0.710743-0.712955 whichwould point to metasedimentary or felsic igneous crustal sources.  Nevertheless their TDM 1.36-1.38 Gais considerably younger than the mean 1.8-1.6 Ga Eastern Sierras Pampeanascrustal residence age and less radiogenic. Middle Devonian magmatism wouldrecord an episode of crustal growth by enriched mantle derived magma input andvariable degrees of partial melting of a lower crustal source at the wanningstages of the Achalian orogeny.