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Rev. mex. cienc. geol vol.28 no.3 México dic. 2011
Tectonothermal history of the Mesoproterozoic Novillo Gneiss of eastern Mexico: support for a coherent Oaxaquia microcontinent
Historia tectonotérmica del Gneis Novillo de edad mesoproterozoica, este de México: soporte para la existencia de un microcontinente Oaxaquia coherente
Robert J. Trainor1,2, R. Damian Nance1,*, and J. Duncan Keppie3
1 Department of Geological Sciences, 316 Clippinger Laboratories, Ohio University, Athens, Ohio 45701, USA.*firstname.lastname@example.org
2 Present Address: 413 Prince Street, Port Hawkesbury, Nova Scotia, B9A 2R2, Canada.
3 Departamento de Geología Regional, Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510, México D.F., Mexico.
Manuscript received: May 25, 2011
Corrected manuscript received: August 22, 2011
Manuscript accepted: August 22, 2011
The Novillo Gneiss is one of several exposures of Mesoproterozoic (ca. 1.0–1.2 Ga) basement in eastern Mexico interpreted to be outcrops of a single crustal block (Oaxaquia) that has figured prominently in continental reconstructions for the late PrecambrianPaleozoic. Exposed within the Sierra Madre Oriental near Ciudad Victoria, the Novillo Gneiss comprises two major Mesoproterozoic igneous suites that intrude rare metasedimentary rocks. The older suite, previously dated at 1235–1115 Ma, principally comprises garnet Kfeldspar augen gneiss and granite gneiss with arc/backarc geochemical affinities. The younger suite (charnokitic gneiss, anorthositic metagabbro) has been dated at 1035–1010 Ma and is interpreted to be part of an anorthositemangeritecharnockitegranite assemblage. Both suites are intruded by two sets of amphibolite dikes, the earlier of which predates metamorphism under granulite facies conditions at ca. 990±5 Ma, whereas the later set is of low grade and was emplaced at ca. 546 Ma.
New structural data in both major igneous suites are dominated by the presence of a composite NWtrending, steeply dipping metamorphic banding/foliation (S1) axial planar to rare isoclinal folds, and a lineation defined by stretched, 12:2:1, Kfeldspar augen and quartz ribbons developed under highgrade metamorphic conditions. The irregular contacts between leucosome and mesosome boundaries may be a remnant of earlier, preS1 migmatization. These are overprinted by tightisoclinal F2 sheath folds associated with a moderately ESEplunging clinopyroxene mineral lineation that bisects the great circle distribution of S and Zshaped asymmetrical F2 fold axes, indicating oblique sinistral, toptoNW relative movement. The L1 lineations also have a great circle distribution suggesting that inhomogeneous stretching in the foliation rotated L1 towards L2. Syntectonic fabrics are overprinted by a granoblastic granulite facies mineralogy followed by retrograde amphibolite and greenschist facies fabrics, the latter associated with the fault that juxtaposes the Novillo Gneiss and the Paleozoic Granjeno Schist. A ca. 350 Ma leucogranite intruded along this fault was deformed at ca. 313 Ma. This tectonothermal history is closely comparable to that of the Oaxacan Complex (southern Mexico), supporting the existence of a coherent Oaxaquia. The absence of the later amphibolite dikes in the Oaxacan Complex likely reflects its relative paleogeography at the time of dike emplacement.
Key words: Novillo Gneiss, Oaxaquia, Mesoproterozoic, Oaxacan Complex, Mexico.
El Gneis Novillo es uno de los varios afloramientos de basamento Mesoproterozoico (ca. 101.2 Ga) en el este de México que han sido interpretados como pertenecientes a un único bloque cortical (Oaxaquia), el cual ha figurado de manera prominente en las reconstrucciones continentales del Precámbrico tardíoPaleozoico. Aflorando en la Sierra Madre Oriental, cerca de Ciudad Victoria, el Gneis Novillo consta de dos suites ígneas mesoproterozoicas mayores que intruyen escasas rocas sedimentarias. La suite más antigua, fechada previamente en 1235–1115 Ma, consiste principalmente de augen gneis de granate y feldespato potásico con afinidad geoquímica de arco/retroarco. La suite más joven (gneis charnokítico, metagabro anortosítico) ha sido fechada en 1035–1010 Ma y se interpreta como parte de un ensamble de anortositamangeritacharnockitagranito. Ambas suites son intruidas por dos conjuntos de diques anfibolíticos, de los cuales el más temprano antecede al metamorfismo en condiciones de facies granulíticas fechado en ca. 990±5 Ma, mientras que el conjunto más tardío es de bajo grado y se emplazó a ca. 546 Ma.
Nuevos datos estructurales de ambas suites ígneas mayores indican que predomina la presencia de bandeamiento/foliación (S1) metamórfico de plano axial con rumbo NW e inclinación de ángulo alto con escasos pliegues isoclinales; la lineación está definida por feldespato potásico y listones de cuarzo alargados, 12:2:1, desarrollada bajo condiciones metamórficas de alto grado. Los contactos irregulares de las fronteras entre el leucosoma y el melanosoma pueden ser un remanente de migmatización más temprana, preS1. Estos son sobrepuestos por ‘sheath folds’ isoclinales cerrados F2, asociados con una lineación mineral de clinopiroxeno con buzamiento moderado al ESE que bisecta las distribuciones de las guirnaldas de los ejes de pliegues asimétricos F2 con forma S y Z, indicando movimiento relativo oblicuo sinestral con movimiento de las capas del techo hacia el NW. La distribución de las guirnaldas de las lineaciones L1 sugiere un alargamiento inhomogéneo que rotó la foliación L1 hacia L2. Las fábricas sintectónicas son sobrepuestas por una mineralogía granoblástica de facies granulítica seguida por fábricas retrógradas de facies de anfibolita y esquistos verdes, la última asociada con la falla que yuxtapone al Gneis Novillo con el Esquisto Granjeno del Paleozoico. Un leucogranito de ca. 350 Ma, que intruyó a lo largo de esta falla, fue deformado a ca. 313 Ma. Esta historia tectonotérmica es muy parecida a la del Complejo Oaxaqueño (sur de México), apoyando la existencia de una Oaxaquia unida. La ausencia de los diques anfibolíticos tardíos en el Complejo Oaxaqueño probablemente refleja su paleogeografía al tiempo del emplazamiento de los diques.
Palabras clave: Gneis Novillo, Oaxaquia, Mesoproterozoico, Complejo Oaxaqueño, México.
The Novillo Gneiss is one of four outcrops of ca. 1.0–1.2 Ga rocks in eastern Mexico (Figure 1) that are interpreted to represent isolated exposures of an extensive Mesoproterozoic basement. Although separated by as much as 700 km, these gneisses (the Novillo Gneiss, Huiznopala Gneiss, Guichicovi Gneiss and Oaxacan Complex) have a number of features in common, including pervasive granulite facies metamorphism, abundant anorthositic complexes, and a NWtrending structural orientation, in addition to their age (OrtegaGutiérrez, 1978; Lawlor et al. 1999; Weber and Köhler, 1999; Keppie et al., 2001, 2003; Solari et al., 2003; Weber and Hecht, 2003; Cameron et al., 2004; Keppie and OrtegaGutiérrez, 2010; Weber et al., 2010).
Early workers considered this basement to be an extension of the ca. 1.0–1.2 Ga Grenville orogen of eastern Laurentia (e.g., de Cserna, 1971; Denison et al., 1971; Shurbet and Cebull, 1987). However, paleomagnetic data (Ballard et al., 1989) and faunal evidence (Robison and PantojaAlor, 1968; Rowley and Pindell, 1989; Stewart et al., 1993) proved inconsistent with this connection and, instead, favored correlation with Gondwana. On this basis, OrtegaGutiérrez et al. (1995) suggested the basement was part of a microcontinent they termed ‘Oaxaquia’, which originated near Gondwana in the early Paleozoic and was only accreted to Laurentia during the late Paleozoic assembly of Pangea. Oaxaquia was inferred to extend from northern Mexico to Central America, a distance of some 2000 km, making it a major segment of the worldwide belt of Mesoproterozoic ("Grenvilleage") orogenesis.
Correlation based on Pb isotope data, however, has been equivocal. Ruiz et al. (1999) provided wholerock data that suggested the northernmost exposure of Oaxaquia (the Novillo Gneiss) isotopically resembled gneisses of the Laurentian Grenville belt in Texas, whereas the Mesoproterozoic gneisses of southern Mexico (the Huiznopala Gneiss, Oaxacan Complex and Guichicovi Gneiss) were isotopically similar to those of the Colombian Andes. This mandated a suture between the Novillo Gneiss and the Huiznopala Gneiss and, hence, brought into question the concept of the Oaxaquia microcontinent. However, Cameron et al. (2004) showed the northern and southern exposures to have similar isotopic signatures based on leached feldspar data, lending support to the concept of a single crustal block, a conclusion since supported by the Hf isotope data of Weber et al. (2010).
To clarify this uncertainty, we present the results of a structuralkinematic and petrographic analysis of the Novillo Gneiss (Trainor, 2010), which we combine with published age data in order to establish a tectonothermal history. We then compare this history with that of the betterknown Oaxacan Complex (Keppie et al., 2001, 2003; Solari, et al., 2003) to the south, the Gondwanan affinities of which are well documented (Landing et al., 2007; Streng et al., 2011). The results support the coherence of Oaxaquia, the existence of which has figured prominently in continental reconstructions for the late Precambrian (e.g., Dalziel, 1997; Li et al., 2008; Scotese, 2009; Weber et al., 2010) and is central to plate tectonic models for GondwanaLaurentia in the Paleozoic (e.g., Keppie et al., 1996, 2008; Keppie and Ramos, 1999).
The Mesoproterozoic (ca. 1.0–1.2 Ga) Novillo Gneiss of northeastern Mexico outcrops in the core of the HuizachalPeregrina anticlinorium (Figure 2), a large NWtrending Laramide structure located in the front ranges of the Sierra Madre Oriental west of the city of Ciudad Victoria (RamírezRamírez, 1974, 1992; OrtegaGutiérrez, 1978).
Occupying an area of about 35 km2, the complex comprises a succession of consistently NEdipping units that includes an intrusive anorthositemangeritecharnockitegranite (AMCG) suite that is flanked to the northeast and southwest by a suite of older orthogneisses and rare metasedimentary rocks (Figure 3). Both suites are intruded by mafic dikes of two generations and are overlain by a Paleozoic cover succession.
The Novillo Gneiss is tectonically juxtaposed to the west against lowgrade Paleozoic rocks of the Granjeno Schist (Figure 2), which make up part of the Sierra Madre terrane. This polydeformed assemblage of metasedimentary and metavolcanic rocks, and serpentinized maficultramafic units is interpreted to be an oceanic accretionary prism possibly associated with the Late Paleozoic closure of the Rheic Ocean (Dowe et al., 2005; Nance et al., 2007). A leucogranite with a poorly constrained UPb zircon age of 351±54 Ma (Dowe et al., 2005) has been emplaced into the fault zone separating the Granjeno Schist from the Novillo Gneiss.
To the east, the Novillo Gneiss is unconformably overlain by an unmetamorphosed Paleozoic (SilurianPermian) sequence of marine clastics (Stewart et al. In ascending order, this sequence includes: (1) Silurian conglomerate, silicic volcanic arenite, limestone, sandstone and siltstone, the latter containing marine fauna of Gondwana affinity, (2) Lower Mississippian shallowmarine, quartz and lithic arenite, siltstone and shale containing Laurentian fauna, (3) Mississippian rhyolite, (4) an unconformably overlying Lower and Middle Pennsylvanian unit consisting of bioclastic grainstone, sandstone, and limy sandstone, and (5) a lower Permian unit consisting of turbiditic siltstone and sandstone rich in volcanic detritus. The Paleozoic succession is unconformably overlain by Mesozoic (Lower JurassicCretaceous) conglomerate, limestone, siltstone and sandstone. CretaceousEarly Cenozoic thinskinned tectonics (Laramide orogeny) have deformed the Mesozoic sequence
above the Jurassic, folding and displacing the overlying strata along NEdirected thrusts (Zhou et al., 2006).
THE NOVILLO GNEISS
UPb zircon dating has shown that the Novillo Gneiss is composed of two major Mesoproterozoic igneous suites (Cameron et al., 2004) that intrude rare metasedimentary (calcsilicate) units. The older suite is dated at 1235– 1115 Ma and principally comprises garnet Kfeldspar megacrystic metagranites and granite gneisses with arc/ backarc geochemical affinities (Cameron et al., 2004). The second suite, dated at 1035–1010 Ma, is composed of charnokitic gneiss and anorthositic metagabbro interpreted to be part of an AMCG suite (Cameron et al., 2004). Both suites were intruded by a set of mafic dikes prior to being metamorphosed under granulite facies conditions dated at ca. 990±5 Ma on the basis of metamorphic zircon growth (Cameron et al. 2004). Peak values for metamorphic pressure and temperature have been estimated at 8.9–9.7 kbar and 730–775 °C (Orozco, 1991).
A second set of lowgrade mafic dikes intrudes all units of the Novillo Gneiss and has yielded a hornblende 40Ar/39Ar plateau age of 546±5 Ma that is considered to closely postdate their emplacement (Keppie et al., 2006). The geochemical signature of the dikes suggests an intraplate magmatic source that these authors link, either to a mantle plume, or to decompression melting during passive asthenospheric mantle upwelling associated with lithospheric extension.
Acooling history for the Novillo Gneiss was estimated by Keppie et al. (2006) on the basis of a UPb titanite age of 928±2 Ma and a 40Ar/39Ar biotite age of 697±10 Ma from a paragneiss. These data suggest that, following granulite facies metamorphism, the Novillo Gneiss cooled from ~660 °C (closure temperature for UPb in titanite; Frost et al., 2000) to 325±25 °C (closure temperature for 40Ar/39Ar in biotite; Harrison et al., 1985) in ca. 230 Ma, a cooling rate of ~1.45 °C/my.
Petrography and textures
Our mapping shows the bulk of the Novillo Gneiss to consist of variably retrogressed granulite facies felsic and mafic orthogneisses. These include garnet Kfeldspar augen gneiss and granite gneiss in the older suite, and charnokitic gneiss and anorthositicmetagabbro units in the younger AMCG suite. Both suites are strongly layered, but whereas the AMCG suite shows continuous, welldefined, rhythmically alternating leucocraticmesocratic layers, layering in the older suite is discontinuous and defined by variably stretched and flattened quartzofeldspathic augen (mmcm in diameter) in a darker foliated matrix. The layering consistently strikes NW and dips SE (Figure 3). Rare calcsilicate units within the older suite are largely massive. Most ferromagnesian minerals show variable alteration to chlorite. The mineralogy is summarized in Table 1.
Rare outcrops of massive calcsilicate in the older igneous suite comprise granoblastic carbonate, euhedral diopside, altered scapolite/wollastonite (with grossular garnet coronas) and recrystallized quartz ribbons.
Older Igneous Suite
A large portion of the older suite comprises monzonitic, garnet Kfeldspar augen gneiss with a prominent blastomylonitic layering. Elongate, asymmetric to symmetric Kfeldspar augen make up the bulk of the leucocratic (granoblastic microclineplagioclasequartz) layers, which are separated by thinner mesocratic bands comprising garnet strings, recrystallized quartz ribbons and pyroxene partially retrogressed to chlorite (Figure 4a).
The granite gneiss of the older suite is granodioritic in composition and shows stromatic leucosome mesosome segregation layering with indistinct layer boundaries. The banding is folded into tight, highly irregular, asymmetric folds on a mmcm scale (Figure 4b). The leucosome comprises approximately equal proportions of granoblastic microcline, plagioclase and rutilated quartz, the latter defining conspicuous recrystallized ribbons that are occasionally isoclinally microfolded. The mesosome contains additional euhedral garnet clusters concentrated along layer boundaries and minor chlorite after pyroxene.
The younger AMCG suite includes strongly layered charnokitic gneiss and anorthositicmetagabbro, but is so pervasively recrystallized to a granoblastic texture (charnokitization) that structures are more easily observed macroscopically than microscopically. Layering in the charnokitic gneiss (Figure 4c) is the result of varying proportions of the same mineralogy, the composition of which is generally monzonitic. Orthopyroxene is usually retrogressed to chlorite, but its presence makes the rock mangeritic or charnokitic.
Layering in the anorthositicmetagabbro is exceptionally coherent, often forming continuous bands across entire outcrops (Figure 4d). The mineralogy is dominated by plagioclase with minor garnet, clinopyroxene (variably altered to chlorite), quartz and opaques, with the darker layers containing more abundant garnet. The layers are cut by veinlets of nelsonite (ilmenite and apatite).
Both suites of orthogneisses are cut by two sets of mafic dikes, the first comprising relict garnet amphibolites, whereas the second comprises porphyritic amphibolite. The older garnet amphibolite dikes are foliated metabasites and range in width on a cmmeter scale. These metabasites are finely laminated with dark green, strongly altered mesocratic layers dominated by fine chlorite (after amphibole and pyroxene) and thin leucocratic laminae containing highly elongate plagioclase lenses and recrystallized quartz ribbons (Figure 4e). The laminations are often strongly folded and cataclastically deformed. A granoblastic texture overprints the foliation defined mainly by garnetrich bands, but a granulite facies mineralogy is no longer preserved.
The younger porphyritic amphibolites are undeformed hornblendeandesites with chilled margins and widths on a dmmeter scale. They display an ophitic texture with randomly oriented plagioclase phenocrysts. The hornblendes contain rare pyroxene cores and are variably replaced by chlorite.
Structure and metamorphism
The AMCG suite occurs as a tectonic slice within the older gneisses; to the west, the ca. 351 Ma leucogranite forms a fault sliver between the Novillo Gneiss and the Granjeno Schist, and to the east, Mesozoic rocks are in fault contact with the Novillo Gneiss (Figure 3). A similar fourfold tectonothermal history is recorded in the older gneisses, the AMCG unit and the garnetiferous dikes (Table 2). The first two events postdate emplacement of the AMCG suite at 1035–1010 Ma, but predate granulite facies metamorphism at ca. 990 Ma, the third coincides with this metamorphism, and the fourth postdates it. The ca. 546 Ma porphyritic mafic dikes postdate all these events.
The dominant fabric in all the rocks is a compositional banding and foliation, S1, with alternating leucocratic (quartzofeldspathic) and mesocratic layers that are interpreted to be a result of highgrade metamorphic differentiation. In places in the granite gneiss, the banding is irregular and stromatic, and is interpreted to be an earlier, preS1, migmatitic feature: this is consistent with garnet concentrations along the borders of quartzofeldspathic leucosome layers resembling migmatite resisters (Winkler, 1979). Rare tightisoclinal folds in the banding have axial planes defined by S1. In the AMCG suite, the S1 foliation is axial planar to tight to isoclinal intrafolial folds that are variably oriented within the steeply NEdipping, S1 foliation (Figure 5a). The pole to the S1 foliation plunges gently SW (statistically at 15°/218°). In the older suite, a prominent lineation, L1, consisting of stretched Kfeldspar augen and quartz ribbons is variably oriented within the S1 foliation (Figure 5b). In the Kfeldspar augen gneiss, the augens are up to 10 cm long and flattened parallel to a weak, steeplyNEdipping foliation developed in the mesocratic layers. Although such fabrics can develop under conditions ranging from greenschist to granulite facies (Passchier and Trouw, 1996; Levine and Mosher, 2010), deformation of the augen requires temperatures above the brittleductile transition in feldspar at ~500°C. The augen are of prolate shape with orientations and length to width to height ratios (X:Y:Z = 12:2:1) that are similar to those of the quartz ribbons. No clear sense of shear is evident in the augen.
The second tectonothermal event in the AMCG unit is recorded by tight to isoclinal, occasionally rootless, asymmetric, Class 1C3 folds (F2), which lack an axial planar fabric and locally refold F1 (Figure 6). Plotted stereographically (Figure 5c), F2 axes are distributed about a great circle, the pole to which plunges gently SW. An associated clinopyroxene mineral lineation (L2) plunges moderatesteeply ESE and suggests continued highgrade metamorphic conditions. The distribution of fold asymmetries (i.e., S and Z folds) is consistent with sheath folding. From exposed sheath fold patterns, the ‘noses’ of which can be used to indicate the direction of transport, folding was the result of sinistral thrusting towards the WNW, parallel to L2. In the older suite, a second planar fabric (S2) is developed axial planar to isoclinal intrafolial microfolds in the metamorphic layering, although both planar fabrics are extensively retrogressed. The quartz ribbons are also locally deformed by isoclinal folds. These folds are gently ESEplunging and have a nearly ideal Class 2 geometry. The long axes of the augen and quartz ribbons in the older gneisses (Figure 5b) also show a similar great circle distribution to that of the isoclinal, F2 fold axes in the AMCG unit (Figure 5c), suggesting that the distribution of the stretching lineation also results from rotation towards the L2 mineral lineation. The absence of an S2 axial planar fabric in the AMCG suite may be due to its massive nature and further development (enhancement) of the S1 foliation.
The garnet amphibolite dikes cut the S1 foliation in the AMCG suite, but are themselves foliated and locally deformed about mesoscopic F2 folds.
The first and second fabrics are pervasively overprinted by granoblastic textures in the gneisses, such that the rocks show no significant strain within either the grains or ribbon structures. This indicates recrystallization under conditions of static metamorphism, the accompanying mineral parageneses of which (orthopyroxene, clinopyroxene and garnet) are indicative of the granulite facies. Because of this, determination of the metamorphic grades of earlier tectonothermal events is difficult.
The final structural/metamorphic event is evident in the development of subgrains and undulose extinction within the granoblastic quartz grains of all lithologies. In the garnet amphibolite dikes, retrograde mineralization and microfracturing associated with this event produced grainsize reduction, ‘birds eye’ feldspars and extensive chloritic retrogression along planes of displacement (S3) that are axial planar to asymmetric F3 folds in both S1 and S2. These folds (Figure 4e) are open, dmscale SWvergent structures with axes that plunge moderately to steeply SE and an axial planar fabric (S3) that dips steeply NE. Offsets along S3 point to a fold mechanism involving oblique shear with a subordinate component of flexural slip indicated by minor cataclastic deformation between compositional layers. The line of movement (perpendicular to the fold axis in the axial plane) plunges moderately northwest (Figure 5d), which in combination with the sense of fold vergence, indicates oblique shear with dextral and reverse components. The majority of microfracture offsets and associated slickensides also indicate dextral shear, consistent with dextral, SWvergent thrusting.
The porphyritic amphibolite dikes display no microdeformational strain features but, instead, preserve original ophitic igneous textures overprinted by retrograde (greenschist facies) mineralization indicating that their intrusion was shallow and postdated all major ductile deformation. Evidence of chilled contacts also indicates that both the older and AMCG suites were at low temperatures when the dikes were emplaced. Some brittle movement is evident along dike contacts, and the dikes are locally offset by minor brittle faults, the majority showing dextral sense. This episode of retrogression must postdate dike intrusion at ca. 546 (Keppie et al., 2006) and is thought to record dextral juxtapositioning of the Novillo Gneiss against the Granjeno Schist (Dowe et al., 2005).
Evolution of the Novillo Gneiss
Based on this tectonothermal record, the Novillo Gneiss is interpreted to record the following history:
(1) Available age and geochemical data (Cameron et al., 2004) show the older suite to have been emplaced between ca. 1235 and 1115 Ma in an arc or backarc setting. Emplacement of the older gneisses may have been accompanied by migmatization, indicating highgrade metamorphism the development of stromatic migmatites requiring temperatures of ~700720oC (Winkler, 1979). Using a 30 oC/km geothermal gradient typical of active continental margins, such migmatization would have required depths of ~25 km.
(2) Emplacement of the AMCG suite occurred between 1035 and 1010 Ma (Cameron et al., 2004).
(3) Highgrade deformation producing two sets of structures in both the older gneisses and the AMCG suite involving the development of a metamorphic differentiation banding/foliation, isoclinal folds, and a 12:2:1 stretching lineation. These were overprinted by sheath folding with sinistralreverse movement parallel to an ESEplunging clinopyroxene lineation under highgrade metamorphic conditions. The garnetiferous mafic dikes cut across the earliest foliation but are themselves deformed by two sets of structures suggesting syntectonic intrusion.
(4) Granulite facies metamorphism (orthopyroxeneclinopyroxenegarnet) replaced all preexisting microfabrics by granoblastic textures. The time of peak metamorphism has been dated at 990±5 Ma on the basis of metamorphic zircon growth (Cameron et al., 2004). Preservation of the preexisting quartz ribbons suggests that the pervasive recystallization was essentially static. The PT conditions of this event have been estimated to be 8.99.7 kbar and 730775oC (Orozco, 1991), the tectonic setting needed to produce the required ~40 km of burial likely being that of the roots of an orogenic belt developed during subduction or collision (e.g., Jamieson et al., 1998).
(5) Initial rapid exhumation must have followed granulite facies metamorphism in order to preserve the
granoblastic textures and some of the peak metamorphic mineralogy. However, on the basis of age data, Keppie et. al. (2006) suggested that the Novillo Gneiss cooled from ~660°C at 928±2Ma to 325±25°C by 697±7 Ma, a relatively slow cooling rate of 1.45 °C/my.
(6) Following exhumation, a suite of porphyritic mafic dikes were intruded at ca. 546±5 Ma (Keppie et al., 2006). Preserved igneous textures and chilled margins indicate that these dikes were emplaced into a relatively brittle, cold host rock. Extrapolation of the 1.45 °C/my cooling rate associated with its exhumation suggests that the Novillo Gneiss would have reached the surface (30 °C) by ~497Ma or 50 my after dike emplacement (Keppie et al., 2006). The ca. 500 Ma exhumation coincides with deposition of the oldest latest Cambrian strata on the Oaxacan Complex (Landing et al., 2007). This suggests that exhumation of the Novillo Gneiss was related to rifting in Oaxaquia.
(7) The final significant tectonothermal event in the Novillo Gneiss is associated with its tectonic juxtapositioning against the Granjeno Schist along the NNWtrending dextral shear zone that separates the two units. The event is manifest in brittle deformation and widespread greenschist facies retrogression of the Novillo Gneiss and the porphyritic mafic dikes. It is most likely to have accompanied development of the weak foliation in the leucogranite emplaced into the shear zone, which has yielded a 40Ar/39Ar age of 313±7 Ma on muscovite, recording cooling through ~350°C (Dowe et al., 2005).
Comparison with the Oaxacan Complex
Broad similarities between the Mesoproterozoic (ca. 1 Ga) exposures in Mexico were instrumental in the proposition by OrtegaGutiérrez et al. (1995) for the existence of the microcontinent "Oaxaquia" as the basement ‘backbone of Mexico’. The largest of these exposures, the Oaxacan Complex (Figure 1), is generally regarded as the Oaxaquia architype (e.g., Keppie, 2004). It is also the southernmost exposure and so is used here to test the continuity of the Mesoproterozic basement (and hence the integrity of Oaxaquia) by comparing its tectonothermal evolution (e.g., Keppie et al., 2001, 2003; Solari et al., 2003) with that of the Novillo Gneiss, the northernmost exposure.
The Novillo Gneiss and Oaxacan Complex share many similarities (Keppie and OrtegaGutiérrez, 2010; Weber et al., 2010). Both comprise older suites of similar age (Novillo Gneiss: 1235–1115 Ma, Cameron et al., 2004; Oaxacan Complex: ≥1350 – ≥1140 Ma, Solari et al., 2003) and similar arc/backarc geochemical signatures that Keppie and OrtegaGutiérrez (2010) suggest reflect their location on the active northern margin (present day coordinates) of Amazonia (Figure 7). The Oaxacan Complex also records migmatization dated at 1106±6 Ma (UPb zircon; Solari et al., 2003) that is designated the Olmecan event. Although the migmatite in the Novillo Gneiss has not been directly dated, it is of similar stromatic character and is interpreted to have accompanied emplacement of the older suite during the interval 1235–1115 Ma.
The older suites of both the Novillo Gneiss and Oaxacan Complex are also intruded by AMCG suites of similar age (Novillo Gneiss: 1035–1010 Ma, Cameron et al., 2004; Oaxacan Complex: 1012–1006 Ma, Solari et al., 2003, Weber et al., 2010), and both underwent granulite facies metamorphism at similar times (Novillo Gneiss: 990±5 Ma, Cameron et al., 2004; Oaxacan Complex: 1004972 Ma, Solari et al., 2003, Weber et al., 2010), followed by exhumation and lowgrade deformation. The highgrade metamorphism is attributed by Keppie and OrtegaGutiérrez (2010) to either arccontinent (protoAvalonia – Amazonia) or, following Li et al. (2008), continentcontinent (Amazonia–Baltica) collision (Figure 7), a scenario favored by Weber et al. (2010).
In addition, both complexes are tectonically juxtaposed against similar Paleozoic sequences (the Granjeno Schist and Acatlán Complex, respectively) that comprise metasedimentary and metavolcanic rocks interpreted to be continental slope or accretionary prism deposits (OrtegaGutiérrez et al., 1999; Dowe et al., 2005; Keppie et al., 2006). The bounding faults in both cases are major dextral shear zones of late Paleozoic age (ElíasHerrera and OrtegaGutiérrez, 2002; Dowe et al., 2005). Finally, both the Novillo Gneiss and Oaxacan Complex are overlain by undeformed Lower Paleozoic strata of Gondwanan affinity (Stewart et al., 1999; Landing et al., 2007).
Strong similarities are also evident in the deformational histories of the Novillo Gneiss and Oaxacan Complex. Both complexes record a similar deformational sequence in which a highgrade, bandingparallel foliation that is axial planar to intrafolial folds is refolded by locally rootless isoclinal folds with variable symmetry and no axial planar fabric (assigned to the 1004–978 Ma Zapotecan Orogeny in the Oaxacan Complex: Solari et al., 2003). Both complexes also contain ubiquitous, mineral lineationparallel, mylonitic quartz ribbons. Given the complex Phanerozoic structural history of southern Mexico, it is not surprising that the present orientation of the structures in the two complexes is different: a steeply NEdipping, composite foliation and variably plunging stretching lineations in the Novillo Gneiss, versus consistently NWplunging stretching lineations and the metamorphic banding/composite foliation deformed about a NWplunging upright fold in the Oaxacan Complex (Solari et al., 2003). Furthermore, the sense of shear in the Novillo Gneiss is oblique, with sinistral and reverse (NE over SW) components, whereas it is SEvergent thrusting in the northern Oaxacan Complex (Solari et al., 2003). However, for the most part, the structures have the same relative orientations, such that the kinematic data for both complexes becomes concordant with a simple ~45° rotation about to pole to their coplanar metamorphic banding. This suggest that their present differences result from local late structural reorientation, although some may reflect decoupling of the middleupper orogenic crust from a 5– 10 km thick, hot (≥700°C), weak lower crust at a depth of ~30 km (Jamieson et al., 1998).
In both the Novillo Geiss and Oaxacan Complex a period of static metamorphism is recorded at the same time (990±5 Ma and 1004–978 Ma, respectively) and was followed by exhumation. Peak metamorphism took place deeper in the Novillo Gneiss (8.9–9.7 kbar; Orozco, 1991) than in the Oaxacan Complex (7.2–8.2 kbar; Mora et al., 1986), and the latter initially cooled much faster (~8 °C/my between 978 Ma and 945 Ma followed by ~2 °C/my: Keppie et al., 2004) than the Novillo Gneiss (~1.45 °C/my) during exhumation.
Like the Novillo Gneiss, the Oaxacan Complex contains mafic dikes that predate granulite facies metamorphism. However, younger dikes equivalent to the porphyritic amphibolites in the Novillo Gneiss have not been reported. Hence, the Novillo Gneiss records an extensional event that is not observed in the Oaxacan Complex. The origin of the porphyritic amphibolite dikes is uncertain. However, Keppie et al. (2006, 2011) have suggested their emplacement could herald the separation of Avalonia from the northern margin of Amazonian, an event thought to be recorded in the Acatlán Complex with the onset of bimodal magmatism at ca. 500 Ma (Keppie et al., 2008). If so, the absence of these dikes in the Oaxacan Complex may simply the northern margin of Amazonian, an event thought to be recorded in the Acatlán Complex with the onset of bimodal magmatism at ca. 500 Ma (Keppie et al., 2008). If so, the absence of these dikes in the Oaxacan Complex may simply reflect a more interior location relative to the developing continental margin.
The Mesoproterozoic Novillo Gneiss is made up of two granulite facies metamorphic igneous assemblages, an older suite dated at 12351115 Ma, which comprises ?migmatitic Kfeldspar megacrystic metagranites and granite gneisses that intrude calcsilicates, and a younger AMCG suite comprising charnokitic gneiss and anorthositicmetagabbro emplaced at 1035–1010 Ma. Both suites are intruded by two sets of mafic dikes – a suite of garnet amphibolites that predate granulite facies metamorphism at ca. 990±5 Ma, and a suite of lowgrade porphyritic amphibolites dated at ca. 546 Ma.
Both suites of gneisses were deformed by polyphase, penetrative fabrics (metamorphic banding, isoclinal folds, stretching and mineral lineations) at high grades of metamorphism culminating in toptotheNW sinistral thrusting. Subsequent granulite facies metamorphism produced ubiquitous granoblastic textures that overprint all earlier microfabrics and was followed by exhumation and the ensuing emplacement of the younger dikes. Dextral thrust juxtapositioning of the Novillo Gneiss against the adjacent Granjeno Schist, a polydeformed assemblage of Paleozoic metasedimentary, metavolcanic and serpentinized maficultramafic rocks, took place in the late Paleozoic and was accompanied by lowgrade metamorphism.
Correlation of the Novillo Gneiss with the Oaxacan Complex of southern Mexico, proposed on the basis of broad similarities in lithologies, age and cover rocks, has been used in previous studies to support the existence of the microcontinent Oaxaquia (Keppie and OrtegaGutiérrez, 2010; Weber et al., 2010 and references therein). Such a correlation is supported by this study, which shows the structural style, deformational sequence and tectonothermal history of the two complexes to be closely similar.
This project was made possible by the Program for North American Mobility in Higher Education with grants from the Fund for the Improvement of Postsecondary Education (to RDN) and the Secretaria de Educación Pública de México (to JDK), and by an Ohio University 1804 award to RDN. RDN and JDK also acknowledge the support of NSF grant EAR 0308105 and PAPIIT grant IN103003, respectively. We thanks Drs. J. Dostal and B. Weber for their constructive reviews of the manuscript.
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