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Orogen-para lle l d isplacemen t of large-scale tecton ic blocks is a common featu re of several orogen ic be lts (c .g . the Himalayas). The phenom enon is genera lly know n as ex trusion tecton ics and occurs in respo nse to stress acco mmoda tion after pla te co llision. Displacement of bloeks or terranes takes place along high-angle shear zo nes that may have the ir roo ts propagated down into the lithospheric mantle, provided a rigid, co upled crus t-ma ntle system is formed , such as in Archaea n cra tons and nuclc ii . If the minimal condi tions for parti al melting o f the mantic arc at taine d, then diamo nd-rich magm as (kimberlite, lamproitc and lamproph yrc) may rise a long or ncar the shear zones and be emplaced at shallow levels in the crust. Th e Pala copro rcrozoic Jtabun a-Salvador-C uracii orogen, Bahia, is a collisional oroge n (c f', Windley 1995) tha t for med by fro nta l or oblique co llis ion of two continents duri ng the Tra nsamazon ian orogeny. At the la titude of Salvado r, th is oro genic belt can he separated into the nor thern and so uthern seg ments. Deta iled description about the latter is g iven in Barbosa (1990, 1997). Th e nor thern seg me nt o f the Itabuna-Sal vador-Curaca orogen (Fig. I), the main su bject o f this co mmunica tion , is thought to have originated by collision of two Archaean blocks, namely the Lcnco is block to the west and the Serr inha nucleus to the east. The latter compri ses migmatites and gneisses of the Santa Luz complex overlain by metamorphosed sed imentary and volcanic rocks of the Rio Itapicuru greenstone belt (RIGI3), later intruded by se vera l types of gra nites and sy enite. Although the Serrinha nucleus and RIGS have been severe ly deformed duri ng the Transamazonian orogen y, two less deformed blocks are recognized , i.e. the Rcriro landia block to the south and the Uau: block 10 the north . According to Chauvct elal. ( 1997 ) RIG B und erwe nt two deformation eve nts. T he older of compressive nature during the clo sure of the sed imentary-volcanic basi n, and the later of N·S ·trending strikeslip nature. Duri ng the latter event , intrus ion o f lamprophyre dykes occurre d in the Mari a Prc tu go ld deposit, and possibly also of small kimberl ite bodies, close to the Qucimada s town to the north . Fart her north, the Uaua Block (3. 16-2.93 Ga. Oliveira et al, 1999) displays a ge neral sigmo id shape and lies in co ntac t by shea r zo nes with the Pa lcop ro tcorozoic Rio Capim metasedimentary-volc anic seq uence to the cas t and the quartzi te-gneiss Caklci riio belt to the west. Th e Uau.i block consists of mafic-u ltrama fic co mplexes, high-grade gneiss and granitoids intruded by at least two malic dyk e swarms. Severa l d iamo ndifemus kimberlite bod ies occ ur a lon g or ncar the contact between the Uauti block and the Rio Capi m Sequence. Field evidence for movement of the Uauri Block moved from south to north is the regional d istribution of the yo ungest mafic dyke swarm refe rred to abo ve and kyncmntic ind icators along the boundary bet ween the Uau.i Block and the Rio Ca pim sequence. Thus, in the ce ntra l region o r the Uauri Block, dykes o f the latter swar m trend mostly to NE -SW. Apar t from minor faults and occ as io nal sheared marg ins these dy kes are not significantly deformed, However, towards the wes tern edge or the Unud Block , the dykes grad ually change d irec tion from NE-SW through N·S to NW-SE until they becom e paralle l to the Caldciri to Belt and di sappear. Some o r these dykes ca n be traced within the transit ion to the Culdc irito Belt where they arc strong ly di srupted or bo udinaged. Metam orph ic titanite gra ins from one o r these dykes yielded the age of 2,039 Ma (Olive ira et al. 20(0). Th e obse rved change o f dyke d irection within the Uaua Block is interpre ted as large-scale drag folds. ind icating a leftlateral di splacem en t o f the Uauti Block in re lat ion to the Ca ldc iriio Belt. On the other hand, the co ntac t between the Palaeoprot e rozo ic Rio Capim seq uence and the Archaean Uaua Block is made through the sub-vertical Gnlo do Ouro shear zone (So uza , 1986). Accordi ng to Ja rdim de S,I et al . ( 198 4) and So uza (1986) this 20-500 Ill-wide shear zone had a complex structural evolution and comprises high-grade mylon itcs with dominantly sub-ho rizonta l mineral stretching lineat ion. Pe tro fubr ic stud ies of quartz c-a xis carried out by Souza ( 1986). co upled with kyn cmatic ind icators observed by this author, Jardim de Sa et al . ( 198 4) and ourselves, such as syn-my lonitic asymmet ric folds, S·C relati on s and mica-fish suppo rt a right-late ral ky ncmatics for the Ga lo do Ou ro shear zone . Sheared mafi c dy kes of the Ua uii Block close to this shear zone yielded a K·Ar age of ca. 2.0 Ga (Bas tes Lea l et al. 1994). All the struc tural and geochronologica l da ta corroborate the synchronous rework ing of bot h western and ea stern margin s of the Uauri Block and its displacement from south to nor th. Although the preci se age of dia mond-bearing bod ies along the contact o f the Uauri Block and the Rio Cap im sequence as we ll as on the wes tern margin of the Rio Itapicuru greenston e belt is not lully constra ined, is possible that the ir emp lace ment was contro lled by co ntine nta l-sc a le shear zones developed duri ng orogc n-parnllc l disp lacemen t o f blocks, such us the Archaea n Uauti block, or later. Area s close to these shear zones

EXTRUSION OF THE ARCHAEAN UAUÁ BLOCK IN THE NORTHERN SEGMENT OF THE ITABUNA-SALVADOR-CURAÇÁ OROGEN, BAHIA, AND IMPLICATIONS FOR DIAMOND PROSPECTING