Notes: Zusammenfassung Neue K-Ar Bestimmungen für die granitoiden Gesteine und Erze aus dem nord-bolivischen Segment (ca. 15°-18° S) der mittleren andeanischen Cordillera Oriental (östlichen Cordillera) bestätigen, daß dieses Gebiet eine beachtenswerte Wiederholung der magmatischen Aktivität und der damit verbundenen Erzmineralisierung aufweist. Granodiorite und teilweise Monzogranite mit Aluminium-Überschuß sind in der Mittel- und Spät-Trias (225-202 Ma) und im Spät-Oligozän bis Früh-Miozän (28.4-ca. 19.2 Ma) eingedrungen; die beiden plutonischen Bereiche liegen mit nur geringer oder fehlender Überlappung benachbart längs einem im wesentlichen linearen Gürtel, der den inneren Rand des mittelandinischen magmatischen Bogen abgrenzt. Es gibt keine entscheidende Anzeichen für granitische Intrusionen in der dazwischen liegenden Zeit, obwohl wenigstens zwei Zentren der oberkretazischen basischen bis intermediären, hypabyssalen bis vulkanischen Gesteinen bekannt sind. Während die triassische Episode gleichzeitig ungefähr 200 km des Gebiets beeinflußte, erfuhr der oligozän-miozäne Bereich jedoch eine longitudinale (südöstliche) Wanderung der Aktivität, die, in den jüngeren Stufen, mit der Einleitung des ausgedehnten Vulkanismus und der Intrusion auf dem Altiplano, westlich des granitoiden Gebietes zusammenfiel. Die beiden intrusiven Gebiete könnte man als die innersten Vorgänge verhältnismägig kurzer Episoden einer drastischen Verbreitung des mittleren andinischen magmatischen Bogens betrachten. Diese könnten vielleicht durch die plötzliche Verringerung des Winkels der östlichen Subduktion an der westlichen Plattengrenze entstanden sein, mit einem Grad der Anatexis, der in den äußeren Teilen des Bogens nicht erreicht wurde. Unsere radiometrischen Daten ermöglichen die Abgrenzung zweier metallogenetischen (W-Sn) Unterprovinzen im nördlichen Teil des bolivischen Zinn-Gebiets. Obwohl die mit den triassischen und tertiären Plutonen verbundenen Wolfram-Zinn-Erzgänge einander sehr ähneln, scheint das Wolfram im älteren Bereich dem Zinn gegenüber etwas angereichert zu sein. Es liegen keine radiometrischen oder petrographischen Zeugnisse für eine erhebliche „Reaktivierung“ von älteren hydrothermalen Systemen im Tertiär vor. Ein spätes früh-miozänes Alter (16.3 Ma) wurde für das wichtige Sn-Ag epithermale Zentrum von Oruro bestimmt. Dieses weicht von der allgemeinen nach S gerichteten Wanderung der vulkanischen und hydrothermalen Aktivität in den zentralen und südlichen „subvulkanischen“ Segmenten der bolivischen Zinn-polymetallischen Gürtel ab, die vonGrant et al. (1979) beschrieben wurde. Geringere Pb-Zn-Ag-Erzgangbildung stand wahrscheinlich mit dem oberkretazischen basisch-intermediären Magmatismus in Verbindung.

Notes: Abstract The Miocene-Pliocene Macusani ash-flow tuffs from SE Peru, containing magmatic andalusite and muscovite, have homogeneous major element compositions, with a narrow range of SiO2 (71–74 wt%), high Al2O3 (normative corundum 〉2%; A/CNK〉1.2) and alkalis, and low FeOt, MgO, CaO, TiO2. P2O5, F, Li2O, and B2O3 are also high. The associated obsidian glasses are more felsic and peraluminous and extremely enriched in F, P, Li and B compared to the ash-flow tuffs. These are compositionally similar to Himalayan or Hercynian two-mica granites and the obsidian glasses to some rare fractionated members of the two-mica granite series. Both ash-flow tuffs and obsidian glasses show enrichments in lithophile trace elements (Be, Zn, As, Rb, Nb, Sn, Sb, Cs, Ta, W, U) and depletions in Cl, S, Sc, V, Cr, Co, Ni, Cu, Y, Mo, Hf. REE patterns for the ash-flow tuffs are fractionated (La/Lu n =13-26) with a moderate Eu anomaly and they contrast with patterns for the obsidian glasses characterized by lower total REE, lower La/Lu n and Eu/Eu*. Sr(87Sr/86Sr initial ratio= 0.721–0.726), Pb (206Pb/204Pb=18.74–19.45; 207P/204Pb= 15.66–15.72) and Nd isotopic compositions (ɛ Nd=-8.96 to-9.35) are typically crustal. Oxygen isotopic compositions are high in 18O (glasses:δ18O=+12‰; quartz:δ18O=+ 11.5 to +12.7‰). Batch melting of isotopically heterogeneous source rocks is suggested by the Sr and Pb data. In contrast to major elements, trace elements demonstrate compositional differences between erupted magmas. The last erupted magmas are less fractionated relative to the first erupted. The Macusani magmas are direct products of crustal melting. There is no evidence for mixing or assimilation by a foreign, meta- to sub-aluminous magma, although mafic magmas are considered to be likely sources of heat for melting. Source rocks dominantly consisted of metapelites. Models of magma generation based on external control of $$a_{H_2 O}$$ (H2O for melting being supplied by aqueous fluids percolating in the source region) fail to account for a number of features of the Macusani magmas. $$a_{H_2 O}$$ was internally controlled and magma generation resulted essentially from fluid-absent melting of F-muscovite combined with incipient biotite dehydration. Fluid-absent melting of F-rich muscovite occurs at higher temperatures than for pure OH-muscovite and generates a H2O-undersaturated melt. Incipient melting of biotite resulted from high heat flux and elevated temperatures (up to 800° C) in the source region. The Macusani magmas are generated as low melt fraction batches (15 vol%).

Notes: Abstract The Miocene-Pliocene Macusani volcanics, SE Peru, outcrop in three separate tectonic intermontane basins developed on a Paleozoic-Mesozoic volcano-sedimentary sequence. Several ignimbrite sheets are recognized and K-Ar dates record at least semi-continuous volcanic activity from 10 to 4 Ma in the Macusani field. The volcanics in the Macusani basin comprise crystal-rich (45% crystals) ash-flow tuffs and rare obsidians glasses, both with unusual mineralogy, similar to two-mica peraluminous leucogranites. The mineralogical assemblage (quartz, sanidine Or69–75, plagioclase, biotite, muscovite and andalusite (both coexisting in the entire volcanic field), sillimanite, schörl-rich tourmaline, cordierite-type phases, hercynitic spinel, fluor-apatite, ilmenite, monazite, zircon, niobian-rutile) is essentially constant throughout the entire Macusani field. Two distinct generations of plagioclase are recognized, viz. group I (An10–20) and group II (An30–45). Sillimanite forms abundant inclusions in nearly all phases and is earlier than andalusite which occurs as isolated phenocrysts. Biotite (Al-, Ti-, Fe- and F-rich) shows pronounced deficiencies in octahedral cations. Muscovite is also F-rich and displays limited biotitic and celadonitic substitutions. There is no systematic variation in mineral chemistry with stratigraphic position. The mineralogical data provide a basis for distinction between an early magmatic and a main magmatic stage. The early stage corresponds to the magmatic evolution at or near the source region and includes both restites and early phenocrysts. Some biotites (with textures of disequilibrium melting to Fe — Zn spinel), part of the sillimanite, apatite and monazite, possibly some tourmaline and cordierite-type phases are restites. However, the restite content of the magma was low (5 vol. % maximum). The group II plagioclase are interpreted as early phenocrysts. During this stage, temperatures were as high as 800° C, pressure was no more than 5–7.5 kbar, $$f_{O_2 }$$ was intermediate between WM and QFM and $$a_{H_2 O}$$ was low. The biotite melting textures and the coexistence of restites and early phenocrysts imply fast heating rates in the source region. The transition between the early and the main magmatic stage was abrupt (andalusite crystallization in place of sillimanite, group I vs. group II plagioclases) and suggests rapid ascent of the magma from its source region. During the main crystallization stage, temperature was 650° C or lower at a pressure of 1.5–2 kbar. $$a_{H_2 O}$$ (calculated from equilibrium between muscovite, quartz, sanidine and andalusite) are around 1, suggesting conditions close to H2O-saturation. f HF is around 1 bar but the $${{f_{H_2 O} } \mathord{\left/{\vphantom {{f_{H_2 O} } {f_{HF} }}} \right.\kern-\nulldelimiterspace} {f_{HF} }}$$ ratios are significantly different between samples. $$f_{H_2 }$$ ranges between 138 and 225 bar. This study shows that felsic, strongly peraluminous, leucogranitic magmas having andalusite and muscovite phenocrysts may be generated under H2O-undersaturated conditions.

Notes: Abstract Mesozoic to Recent volcanic rocks from a transect of the Central Andes between latitudes 26 ° and 28 ° South in northern Chile and Argentina show chemical and temporal zonation with respect to the Peru-Chile trench. Jurassic to Eocene lavas occur closer to the trench and are comparable to calc-alkaline rocks of island arcs. Eastwards they are followed by Miocene to Quaternary sequences of typical continental margin calc-alkaline rocks which have higher contents of K, Rb, Sr, Ba, Zr, and REE and also higher K/Na and La/Yb ratios. The rocks occurring farthest from the trench have shoshonitic affinities. The distribution of major and trace elements is consistent with a model in which magmas were derived by anatexis of an upper mantle source already enriched in LILE and located above the descending oceanic slab. It is suggested that the chemical variations across the volcanic belt reflect systematic changes in the composition of the magmas due to a decreasing degree of partial melting with increasing depth, and probably also due to the heterogeneity of the source materials.