A Microstructural and Petrologic Study Of the Scarboro Formation Casco Bay Sequence South-Central Maine
Bibliographic record
Abstract
Ordovician rocks of the Liberty-Orrington belt in south-central Maine are flanked by younger rocks to the southeast (Fredericton belt) and northwest (central Maine belt) and thus occupy an important structural position in the region. This study focuses on detailed petrologic and microstructural analyses of Middle Ordovician (?) metapelitic rocks of the Scarboro Formation (upper part of the Casco Bay sequence) exposed within the Liberty-Orrington Belt. At least two phases of deformation are observed in these rocks: (1) An early phase of upright isoclinal folding with an associated axial planar foliation that trends N10-25ºE and dips steeply (> 60º) to the southeast. (2) A later phase (based on cross-cutting relationships) of dextral shear deformational features. Metamorphic mineral assemblages in these rocks indicate low-pressure metamorphism (< Al2SiO5 triple point conditions) at lower sillimanite grade conditions. Typical AFM mineral assemblages in these rocks include garnet, biotite, andalusite and sillimanite. Local variations in bulk composition result in the presence of staurolite (Ferich) and cordierite (Mg-rich) porphyroblasts in some samples. Compositional data from garnets indicate high manganese concentrations (up to 30 mole % in cores) and patterns typical of growth zoning. Conditions of peak metamorphism are established to be 540- 590ºC and 2.4-3.0 kilobars based on petrogenetic grid constraints. Previously published Ar-Ar hornblende ages provide a Middle Devonian age for the timing of this metamorphism. Microstructural analysis of porphyroblast-matrix relationships in these rocks provides information on the timing of metamorphic mineral growth relative to the deformational fabrics in the rocks. Complex, locally spiraled inclusion trails in garnet porphyroblasts are generally discordant to the matrix foliation. Caution should be taken in interpreting such relationships, but it is suggestive of growth during a deformational episode that preceded the development of the matrix foliation. Andalusite and staurolite porphryoblasts are often syn-tectonic with respect to the external foliation, although examples of post-tectonic growth can be found. This external foliation is parallel to the axial surfaces of upright isoclinal folds and thus peak metamorphic mineral growth appears to have been synchronous with this deformational episode. Localized, late dextral shear deformation of all porphyroblasts often accompanied by chlorite growth indicates this phase of deformation occurred after peak metamorphism (Middle Devonian).
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.002 | 0.001 |
| Science and technology studies | 0.001 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
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