Tamboril-Santa Quiteria Complex, Ceara, Brazil
|
|
This page documents our findings in and around the Santa Quiteria granite body. The observations were made in a triangular zone between the towns of Itapage, Itapipoca and Forquilha (map in Fig. 1). The granite complex is comprised of a porhyritic granite with mafic enclaves and has been dated at 640Ma. It is surrounded by migmatitic rocks including anatectic facies of the Santa Quiteria granite and older orthogneisses and paragneisses. This series include eclogite (paleo-eclogite bodies) seen in pt 106. The metamorphic grade of the migmatites to the south of the granite body increases with distance. First, there is a migmatite band which lacks anhydrous peritectic minerals and suggest water-fluxed, low-T melting. This is followed southwards by the appearnace of garnet in the leucosomes and an increase in melt fraction represented by leucosomes, and indicative of Bt-dehydration melting. Interestingly, we found porphyritic granite very similar to the Santa Quiteria pluton, migmatized in both regions, forming a garnet, K-feldspar porphyritic migmatite at high metamorphic grade. Migmatization has been dated at 620Ma, 20 m.yr. after granite crystallization. Structurally we investigated the west part of the area close to Forquilha where the regional trends are dominantly N-S and a band south of the pluton including the region of Itapage and to the south. In the E=W trending region close to Forquilha, we found normal movement on C=010/30E (top to the E) overprinted by dextral movement on C=010/80E in migmatites with garnet in leucosome (pt 104), or normal movement on C//So=20/30E also in migmatites with garnet in leucosomes (pt 119) In the region south and immediately west of Santa Quiteria pluton, the rock sequence comprises mylonites, protomylonites and gneisses (ortho and para), and the main trend in E-W and gently N-dipping. Southwards from the pluton the movement sense changes from recording strike slip movement to recording north-dipping normal movement. The normal shearing found in Forquilha and south of Itapage is surprising given what is reported in the literature. It points to the possibility that the core of the pluton went down in relation to its surroundings during a generalized extension. This explains the exhumation of the 650Ma eclogites.
|
|
| a) Geological Map of the Santa Quiteria granite in red and surroundings with hand written field stations. |
|
| b) Block of Kfs porphyritic, biotite, Santa Quiteria Granite, with shear band diagonally across the photograph. pt 211. |
|
|
| c) Sheared, porphyritic granite, anatectic (Hbl leucosome) and partly transposed, preserving an old, folded foliation, main transposition plane N10-20E with in source granite and pegmatite. No anhydrous peritectic minerals. pt 511. Interpreted to be a Sta Quiteria granite diatexite. | d) Same as (c) |
|
|
| e) Migmatitic gneiss after Sta Quiteria granite with peritectic Hbl. No anhydrous peritectic minerals. pt 246 C= N30E/20NW, Lx=270/07 260/10 top-to-west shear sense. | f) Migmatitic gneiss with peritectic Hbl. No anhydrous peritectic minerals. 830Ma orthogneiss dated at this locality, C=095/40N. Lx=015/40 no good shear sense. Pt. 221. |
|
|
g) Diatexitic migmatite in paragneiss interlayered with amphibolites. Amphibolite layers (5cm thick) control folding and boudinage. No anhydrous peritectic minerals indicate paragneisses undergoing water fluxed melting anatexite. Pt 164 Road to Jua, southwards from BR222, close to contact with Sta Quiteria body. . . | h) Diatexitic migmatite after paragneiss interlayered with amphibolites. Notice folded and disaggregated resistate. No anhydrous peritectic minerals. |
|
|
i) Diatexitic migmatite after paragneiss interlayered with amphibolites. Folded layer with leucosome in boudin neck. No anhydrous peritectic minerals. . . | j) Diatexitic migmatite after paragneiss. Notice intensive disaggregation of resistate. No anhydrous peritectic minerals. |
|
|
k) Garnet bearing (Bt rims) leucosome in Sta Quiteria porphyritic granite, sheared and melted (Bt dehydration). Pt 104 Cachoeira das Mocas, north of BR222. Lineation 95/20 two movement senses to east and west, Bt-dehydration melting of Sta Quiteria Granite . . | l) Garnet-bearing migmatite Santa Quiteria porphyritic granite dextrally sheared on N10E/svert at the margins of a pegmatite (horizontal plane). Same locality as (i). Pt 104. |
|
| m) Eclogite: Gnt+Amph+Cpx within paragneissic migmatites with lots of garnet (17 kbar). Pt 106. |
Deformation
|
|
| a) Photograph parallel to lineation Lx=040, normal movement on vertical wall: top-to-the-right (northeast). Acude Gerimum pt 15. This is the rock enveloping the lense of Gnt-Bt-Sil rock. | b) Photograph parallel to lineation Lx=040, normal movement on vertical wall: top-to-the-right (northeast). Acude Gerimum pt 15. This is the rock enveloping the lense of Gnt-Bt-Sil rock. |
|
| c) Normal movement, top-to-the-right on 065/20N, Lx = 00/20. Hbl-Gnt bearing migmatite of paragneiss (same outcrop as Fig. 1 (e) and (f) above). pt 219. This is east of point 15 Acude gerimum where it is also top to north normal |