Petrography
The studied metadolerite dikes show various textures in response to the extent of dolerite recrystallization and deformation (Table 1). These textures range from intersertal or intergranular and blastophitic to mylonitic and cataclastic. Metadolerite dikes with intersertal/intergranular and blastophitic texture consist of plagioclase, clinopyroxene, and opaque minerals (Fig. 4a), which represent the primary (igneous) minerals. Plagioclase is commonly sericitized or saussuritized or is completely replaced by fine-grained aggregates of pumpellyite, prehnite, chlorite, and epidote. Fresh clinopyroxene occurs as twinned or zoned crystals showing undulose extinctions. More often clinopyroxene is partially replaced by chlorite, white mica, and various types of amphibole (mainly brown-, green-, and blue-amphibole). Brown amphibole is present as single crystals (Fig. 4a), as pseudomorph after clinopyroxene (Figs. 4a-4d), or it forms coronas rimming clinopyroxene. Green amphibole crystallized as single crystals, as pseudomorph after clinopyroxene, or it forms coronas rimming brown amphibole or clinopyroxene (Figs. 4c, 4d). Pale-green amphibole may form pseudomorphs after clinopyroxene (together with chlorite and white mica). Most commonly, uncoloured or pale-green amphibole rims the brown amphibole and is in turn rimmed by blue-green amphibole (Fig. 4b). Blue amphibole commonly rims the brown amphibole or the green amphibole (Fig. 4c). On the basis of amphibole type and texture, several occurrences were distinguished and summarized in Figure 5.
Metadolerite with mylonitic and cataclastic textures characterize the most deformed domains. In these rocks, the primary mineralogy and structure have been obliterated, with the exception of porphyroclastic clinopyroxene which displays a sigmoidal shape.
Figure 4. Photomicrographs of metadolerite dike (sample MC81).
a) General view of the rock showing intersertal structure (altered plagioclase and clinopyroxene), cut by a vein filled with chlorite+epidote. Brown amphibole occurs as single crystal (upper left) or partly replacing clinopyroxene. Plane polarized light. b) Enlargment of fig. 4a showing brown amphibole (replacing clinopyroxene) rimmed by pale-green and blue amphiboles. Plane polarized light. c) Altered plagioclase, brown amphibole (replacing clinopyroxene) rimmed by green amphibole which is rimmed by blue amphibole. Plane polarized light. d) Brown amphibole grading to green amphibole rimmed by blue amphibole growing in the textural site of clinopyroxene. Chlorite grows at the core of the cpx site. Plane polarized light.
Figure 5. Simplified sketch of amphiboles in metadolerite samples.
Simplified sketch summarizing the textures of amphiboles crystallized during oceanic and subduction-related metamorphism in metadolerite samples with intersertal/intergranular and blastophitic textures (see text for explanation and mineral abbreviations).
Additional metamorphic minerals were observed in all metadolerite types and include quartz, albite, titanite, apatite, and typical blueschist facies minerals such as blue-amphibole and lawsonite (Table 1). Metadolerites are commonly cut by veins filled with metamorphic minerals as pumpellyite, chlorite, prehnite, albite, tremolite/actinolite, white mica, quartz, calcite, epidote, lawsonite, glaucophane, and chrysotile, which occur with various combinations within the same vein (see also Sansone et al., 2011; Sansone and Rizzo, 2012).
Petrographic and microstructural observations carried out on metadolerite samples allowed us to reconstruct the textural and metamorphic evolution of dolerite dikes intruding serpentinites in the Frido Unit. The results are summarized in Table 2, which refers to the entire set of investigated samples, although we focused our analysis on metadolerites characterized by intersertal/intergranular and blastophitic texture. In particular, one sample (sample MC81; Table 1) was selected for the description of pseudomorphic and coronitic domains. These domains (as small as about 500µm) trace in detail the mineralogical and textural changes induced by oceanic and subduction-related metamorphism. The results are illustrated and discussed after presentation of the mineral chemistry data.