The Sasso di Castro ophiolite is a huge olistolith within the chaotic complex (late Cretaceous) of the external Liguride unit at Futa Pass, Tuscan Apennines. The ophiolite consists of massive basalts cut by few dykes and overlain by pillow basalts which grade up into pillow basalt breccia covered by red cherts. The whole sequence was affected by ocean-floor metamorphism under static conditions from lower amphibolite to zeolite facies. As opposed to other occurrences in the Northern Apennines, the Sasso di Castro plagiogranites (trondhjemites and very scarce diorites) occur as two small stock-like bodies intruded in the lower part (massive basalt) of the effusive complex. The largest intrusion produced a small thermometamorphic aureole whose hornfelses reached lower amphibolite facies. The thermal effect was associated with fragmentation of the wall-rock and locally also with plastic deformation. Chemical exchange between the host massive basalts and the intruded plagiogranite body has been recognized. The plagiogranites, particularly the trondhjemites, have extremely low K2O (<0.15%) and CaO (<0.35%) contents as well as a high Na2O/K2O (>50) ratio. However the presence of a small amount of modal biotite indicates crystallization from a relatively K2O-rich 'granitic' liquid. Low K2O and CaO contents and absence of K-feldspar in the Sasso di Castro plagiogranite probably result from K/Na and Ca/Na exchange in feldspars related to hydrothermal ocean-ridge metamorphism. All the analysed basalts were sampled closely to one another in a quarry. The ratios between refractory elements such Ti, Zr, Nb, Y allow the subdivision of the Sasso di Castro basalts into two groups with high and low Zr/Y; they can be interpreted as two series fractionated from two different parental magmas, which are more enriched in incompatible element than normal MORB. Melting models of the Sasso di Castro basalts as well as of basaltic rocks from Northern Apennines, Western Alps and Corsica ophiolites point to at least two significantly different types of mantle sources: one LREE enriched (Maggiorasca, external Ligurides, Montgenevre, Balagne), the other LREE depleted (Inzecca, Eastern Liguria). It is shown that the Western Mediterranean ophiolite basalts (which are more enriched in incompatible elements than normal MORB) are clearly distinguishable from both transitional and enriched MORBs
Geology and petrology of the Sasso di Castro ophiolites and associated plagiogranites.
MARRONI, MICHELE;
1987-01-01
Abstract
The Sasso di Castro ophiolite is a huge olistolith within the chaotic complex (late Cretaceous) of the external Liguride unit at Futa Pass, Tuscan Apennines. The ophiolite consists of massive basalts cut by few dykes and overlain by pillow basalts which grade up into pillow basalt breccia covered by red cherts. The whole sequence was affected by ocean-floor metamorphism under static conditions from lower amphibolite to zeolite facies. As opposed to other occurrences in the Northern Apennines, the Sasso di Castro plagiogranites (trondhjemites and very scarce diorites) occur as two small stock-like bodies intruded in the lower part (massive basalt) of the effusive complex. The largest intrusion produced a small thermometamorphic aureole whose hornfelses reached lower amphibolite facies. The thermal effect was associated with fragmentation of the wall-rock and locally also with plastic deformation. Chemical exchange between the host massive basalts and the intruded plagiogranite body has been recognized. The plagiogranites, particularly the trondhjemites, have extremely low K2O (<0.15%) and CaO (<0.35%) contents as well as a high Na2O/K2O (>50) ratio. However the presence of a small amount of modal biotite indicates crystallization from a relatively K2O-rich 'granitic' liquid. Low K2O and CaO contents and absence of K-feldspar in the Sasso di Castro plagiogranite probably result from K/Na and Ca/Na exchange in feldspars related to hydrothermal ocean-ridge metamorphism. All the analysed basalts were sampled closely to one another in a quarry. The ratios between refractory elements such Ti, Zr, Nb, Y allow the subdivision of the Sasso di Castro basalts into two groups with high and low Zr/Y; they can be interpreted as two series fractionated from two different parental magmas, which are more enriched in incompatible element than normal MORB. Melting models of the Sasso di Castro basalts as well as of basaltic rocks from Northern Apennines, Western Alps and Corsica ophiolites point to at least two significantly different types of mantle sources: one LREE enriched (Maggiorasca, external Ligurides, Montgenevre, Balagne), the other LREE depleted (Inzecca, Eastern Liguria). It is shown that the Western Mediterranean ophiolite basalts (which are more enriched in incompatible elements than normal MORB) are clearly distinguishable from both transitional and enriched MORBsI documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
