![]() In order to account for variation of magma composition in time and space, we envisage a transitional MASH zone which has responded to crustal thickening since the Upper Miocene, involving breakdown of mineral phases such as plagioclase and amphibole, and/or variable degrees of partial melting of the crust. However, for a number of unusual basalts and basaltic andesites in small and rare isolated monogenetic centres, MASH homogenization is not indicated and is in conflict with chemical characteristics. chemical and isotopie characteristics that are constant throughout the entire compositional range of an individual volcanic structure. Homogenization of deeply-derived contaminated magmas produced typical “MASH-baseline values”, i.e. Crust-magma interaction is viewed as a MASH-type process (melting, assimilation, storage, homogenization) in lower crustal melting regions. Together these are interpreted as clear evidence that isotopie (and thus bulk chemical) compositions are strongly affected by magma-crust interaction rather than by changes in the mantle source. There is also a striking correlation between the variation in isotopie composition of erupted magmas through time and the crustal uplift/crustal thickening history of this region. The Alto de Pica sector is also unusual in that it lacks the tectonic horst of the Western Cordillera and may correlate with a major transverse crustal domain boundary. Ollague where AFC trends are observed (4) Sr- and, in particular, Pb - isotopic compositions which differ north and south of the Alto de Pica area at about 19.5°S, and reflect interaction of magmas with domains of different crustal type and mean crustal age and (5) interruption of the volcanic chain north and south of the Alto de Pica area where recent volcanism is absent over 180 km in a N-S direction, except for a single rhyodacite mega-dome (Co. Variations along the N-S chain of the volcanic front include: (1) regional segmentation into zones of abundant and voluminous volcanic complexes that evolve from rare basaltic andesites to rhyodacites and rare rhyolites (2) northern centres with higher incompatible trace element contents than southern ones (3) constant isotopic composition with differentiation for most centres, with the exception of V. In addition, the crustal thickness has varied through time from around 40 km in the Lower Miocene to about 70 km today. The only significant variable along the current arc segment is in mean crustal age (Palaeozoic in the south to Proterozoic in the north). Crustal thickness, distance from the trench, height above the seismically active subduction zone, and sediment supply to the trench are all constant along this segment of the arc. Geochemical data from 37 volcanic centres from the active volcanic front in the Central Andes between 17.5° and 22°S of northern Chile provide constraints on crustal contributions to arc magma genesis in that region. Besides it shows the historical development of a medical phenomenon up to the present, which has been strongly influenced by the cultural conditions of the respective societies. This paper gives an overview of Pica as a typical disease of women between the 16th and 18th centuries. The consumption of things commonly considered as inedible is seen as a very rare symptom that can be found mainly with people with psychological problems or mental deficiencies. Contrary to former times, today the medical and social interest in strange cravings is almost insignificant. There were new perceptions of this eating disorder among the doctors and its importance declined. In the course of time the view of Pica changed. Their publications reflect the commonly accepted medical knowledge of that time, religious beliefs and prevailing ideas as to possible therapies. ![]() Contemporary doctors developed theories about the disease and its genesis. At that time cravings for substances like chalk, soil or paper were discussed as a medical condition typically affecting young girls and pregnant women. It also was the subject of numerous medical treaties and intensive scientific research. Between the 16th and 18th centuries it aroused widespread popular interest. Pica, or the intentional consumption of things commonly considered as inedible, is a very old phenomenon.
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