STRUCTURAL, GEOCHEMICAL AND GEOCHRONOLOGICAL ANALYSIS OF
THE COATLACCO AREA, ACATLÁN COMPLEX, SOUTHERN MEXICO
A thesis presented to
the faculty of
the College of Arts and Sciences of Ohio University
In partial fulfillment
of the requirements for the degree
Master of Science
Kathryn R. Grodzicki
August 2006
This thesis entitled
STRUCTURAL, GEOCHEMICAL AND GEOCHRONOLOGICAL ANALYSIS OF
THE COATLACCO AREA, ACATLÁN COMPLEX, SOUTHERN MEXICO
by
KATHRYN R. GRODZICKI
has been approved for
the Department of Geological Sciences
and the College of Arts and Sciences by
R. Damian Nance
Professor of Geological Sciences
Benjamin M. Ogles
Dean, College of Arts and Sciences
Abstract
GRODZICKI, KATHRYN R., M. S. August 2006. Geological Sciences
STRUCTURAL, GEOCHEMICAL AND GEOCHRONOLOGICAL ANALYSIS OF
THE COATLACCO AREA, ACATLÁN COMPLEX, SOUTHERN MEXICO (99 pp.)
Director of Thesis: R Damian. Nance
The Acatlán Complex of southern Mexico's Mixteca terrane is a large inlier of
Paleozoic rocks interpreted to be a vestige of the Iapetus and/or Rheic Oceans. Critical tests for this linkage are provided by the depositional age of the siliciclastic Cosoltepec
Formation and the geochemical affinity of associated mafic volcanic rocks.
Siliciclastic metasedimentary and mafic metavolcanic rocks near Coatlacco,
Guerrero State, southern Mexico make up two units: (i) interbedded psammites and pelites recording up to four phases of deformation that can be correlated with the type
Cosoltepec Formation, and (ii) quartzite, with locally pillowed mafic metavolcanic rocks here named the Coatlacco unit, which record only two phases of deformation. U-Pb detrital zircon dating of the metasediments and within-plate affinities of the basalts are consistent with deposition and volcanism related to Late Devonian – Mississippian continental extension.
Approved:
R Damian. Nance
Professor of Geological Sciences
Acknowledgments
I would like to thank a number of individuals who assisted me with the completion of this thesis. Firstly I would like to thank R. Damian Nance for giving me the opportunity to undertake this research and for sharing his knowledge with me. I would also like to thank J. Duncan Keppie of U.N.A.M, Mexico City for introducing me to the geology of Mexico and supplying field equipment. I would also like to thank thesis committee members Julie Libarkin and especially David Schneider, for their assistance and suggestions in the improvement of this thesis. Thank you to Jaroslav Dostal and
Brendan Murphy for assistance with interpreting geochemical data, Victor Valencia and
George Gehrels for assisting with detrital zircon analysis and Chuck Bennett and especially Greg Nadon for their technical assistance. Thank you to Brent Barley and my field partners Kyle Shalek and especially Hector Hinojosa, who also assisted with detrital zircon analysis. 5 Table of Contents
Abstract…………………………………………………………………………………. 3
Acknowledgements……………………………………………………………………... 4
List of Tables………………………………………………………………………….... 8
List of Figures…………………………………………………………………………... 9
List of Plates……………………………………………………………………………. 12
Introduction……………………………………………………………………………... 15
Geology of Mexico……………………………………………………………………... 16
Mixteca terrane and the Acatlán Complex………………………………………... 16
Mixteca terrane………………………………………………………………... 16
Stratigraphy……………………………………………………………………. 17
Cosoltepec Formation……………………………………………………... 20
Petlalcingo Suite…………………………………………………………... 21
Chazumba Formation…………………………………………………… 21
Magdalena Migmatite…………………………………………………... 22
Tecomate and Patlanoaya Formations…………………………………….. 22
Piaxtla Suite……………………………………………………………….. 23
Geology of the Coatlacco area………………………………………………………….. 24
Field relations……………………………………………………………………... 24
Cosoltepec Formation…………………………………………………………. 28
Coatlacco unit…………………………………………………………………. 29
Piaxtla Suite…………………………………………………………………… 31 6 Cualac Conglomerates………………………………….……………………... 32
Undeformed Volcanic Rocks……...…………………………………………... 33
Summary………………………………………………………………………. 33
Structure and Petrography……………………………………………………………… 34
Cosoltepec Formation……………………………………………………………... 34
Psammites……………………………………………………………………... 34
Pelites………………………………………………………………………….. 37
Metabasic rocks……………………………………………………………….. 41
Coatlacco unit……………………………………………………………………... 42
Quartzites……………………………………………………………………… 42
Mafic lithologies………………………………………………………………. 44
Comparison with the type Cosoltepec Formation………………………………… 46
Piaxtla Suite……………………………………………………………………….. 48
Amphibolites…………………………………………………………………... 48
Granitoids…………………………………………………..…………………. 49
Geochemistry…………………………………………………………………………… 51
Whole – rock geochemistry……………………………………………………….. 51
Interpretation……………………...……………………………………………….. 53
Comparison with the type Cosoltepec Formation………………………………… 57
Geochronology…………………………………………………………………………. 58
Cosoltepec Formation……………………………………………………………... 60
Coatlacco unit………………………...... 60 7 Comparison with the Cosoltepec Formation Type Section……………………….. 62
Interpretation………………………………………………………………………. 63
Discussion………………………………………………………………………………. 68
Conclusions...…………………………………………………………………………… 72
References………………………………………………………………………………. 75
Appendix A……………………………………………………………………………... 81
Appendix B……………………………………………………………………………... 92
Appendix C……………………………………………………………………………... 94
8 List of Tables
Table Page
1: Geochemical data for basalt samples from Coatlacco and La Cueva, Acatlán Complex, Mixteca terrane, southern Mexico………………………………………. 95
2: Geochemical data of Keppie et al. (in press) for basalts from the Cosoltepec Formation, Acatlán Complex, Mixteca terrane, southern Mexico…………………. 96
3: Detrital zircon data for psammites of the Cosoltepec Formation (sample LCM 3), Coatlacco area, Acatlán Complex, Mixteca terrane, southern Mexico…………. 97
4: Detrital zircon data for quartzites of the Coatlacco unit (sample LCB 3), Coatlacco area, Acatlán Complex, Mixteca terrane, southern Mexico…………….. 98
5: Structural data for psammites and pelites of the Cosoltepec Formation and quartzites of the Coatlacco unit, Coatlacco area, Acatlán Complex, Mixteca terrane, southern Mexico………………………………………………………….... 99
9 List of Figures
Figure Page
1: Tectonostratigraphic setting of the Acatlán Complex, Mixteca terrane, southern Mexico. Guerrero, Cuicatecco and Chatino terranes are Mesozoic whereas the Oaxacan and Maya terranes have Grenville-age (ca. 1.0 Ga) basement. CF = Caltepec Fault, CAF = Chacalapa Fault, PT = Papalula Thrust (modified after Ortega-Gutiérrez et al., 1999 and Keppie et al., 2004b)…………………………… 17
2: Tectonostratigraphy of the Acatlán Complex comparing the traditional interpretation of Ortega-Gutiérrez et al. (1999) with the recently revised tectonostratigraphic interpretation of Nance et al. (2006)…………………………. 19
3: Geological map of the northern half of the Acatlán Complex modified after Ortega-Gutiérrez et al. (1999). Map shows main lithological units along with the study area of Coatlacco east of the town of Olinalá……………………………….. 25
4a: Geological map of the Coatlacco area, Acatlán Complex, Guerrero State, southern Mexico. Field map is presented in Appendix B………………………….. 26
4b: Key to field map of the Coatlacco area and location map for the field area with the main town of Cualac marked for reference………………………………. 27
5: Equal area stereographic projection of contoured poles to SCF1, SCF2 and SCF3 in the Cosoltepec Formation of the Coatlacco area, Acatlán Complex, Guerrero State, southern Mexico. Fabrics are generally relatively steeply dipping to the NW and SE, trending NE-SW. SCF1, n=104; SCF2, n=160; SCF3, n=6…………….. 40
6: Equal area stereographic projection showing the orientation of FCF1, FCF2, FCF3 and FCF4 fold axes in psammites and pelites of the Cosoltepec Formation of the Coatlacco area, Acatlán Complex, Guerrero State, southern Mexico. FCF1, FCF2 and FCF4 axes generally trend NE-SW, while FCF3 is more variable in its plunge… 40
7: Equal area stereographic projection of contoured poles to SCF1, SCF2 and SCF3 and their relationship to FCF1, FCF2, FCF3 and FCF4 axes of psammites and pelites in the Cosoltepec Formation of the Coatlacco area, Acatlán Complex, Guerrero State, southern Mexico. SCF1, SCF2 and SCF3 are axial planar to FCF1, FCF2, FCF3 and FCF4. SCF1, n=104; SCF2, n=160; SCF3, n=6………………………………………… 41
8: Stereographic plot of poles to SCU1 and SCU2 foliations in quartzite of the Coatlacco unit, Acatlán Complex, Guerrero State, southern Mexico. Fabrics are relatively steeply dipping to the SE……………………………………………….. 43
10
9: Zr/TiO2 vs. SiO2 discrimination diagram for metabasalts from this study (in pink) and those of Keppie et al. (in press) (in blue). Basalts plot generally as sub- alkaline basalts with a single sample from this study and two samples from Keppie et al. (in press) plotting as andesites………………………………………. 52
10: Plot of Cr vs. Ni showing variable degrees of fractionation in basalts from this study (pink) and those of Keppie et al (in press) (blue). Arrow shows the direction of increasing fractionation…………………………………..……………………... 52
11: Ti-Zr-Y discrimination diagram for basalts from this study (pink) and from those of Keppie et al. (in press) (blue) (after Pearce and Cann, 1973). Samples plot as within-plate basalts and a single sample from this study plotting in the MORB – island-arc – calc-alkali field..….……………………………………….... 54
12: Plot of Cr vs. Ti for basalts from this study (pink) and those of Keppie et al. (in press) (blue). Basalts plot as low potassium tholeiites…………………………. 54
13: Plot of FeO/MgO vs. SiO2 (after Miyashiro, 1974) places basalts on the boundary of between calc-alkalic and tholeiitic compositions with a single sample from this study (pink) and two samples from Keppie et al. (in press) (blue) plotting as tholeiitic basalts………………………………………………………... 55
14: Plot of FeO/MgO vs. FeO (after Miyashiro, 1974) places samples from this study (pink) and those from Keppie et al. (in press) as tholeiitic basalts………….. 55
15: Primitive mantle normalized abundance patterns for basalts from the Coatlacco and La Cueva area plotted with average element concentrations for MORBs, OIBs and continental basalts (after Rollinson, 1994). Colored lines are data from this study while yellow polygon shows the range of data of Keppie et al (in press)…………………………………………………………………………… 57
16: Cumulative probability plot and Concordia diagram (inset) for psammites (sample LCM 3) from the Cosoltepec Formation of the Coatlacco area taken along Aconcingo stream west………………..…………………………………….. 61
17: Cumulative probability plot and Concordia diagram (inset) for quartzite (sample LCB 3) from the Coatlacco unit taken along the Coatlacco River at La Cueva………….……………………………………………………….…………... 62
11
18: Detrital zircon age populations in the Acatlán and Oaxacan complexes compared to age provenances in eastern Laurentia, Baltica, west Avalonia, Amazon Craton and the West African Craton. Colored bars show age populations for this study and that of the type Cosoltepec Formation. Xayacatlán and Tecomate populations are those of Talavera-Mendoza et al. (2005) (after Nance et al., 2006 and Murphy et l., in press)……………………...... 64
19: Paleocontinental reconstruction for Late Ordovician – Early Silurian after Ortega-Gutiérrez et al. (1999). Model envisages the Acatlán Complex to be part of the Iapetus suture formed by the docking of Oaxaquia against the southern margin of Laurentia……………...………………………………………………… 65
20: Paleocontinental reconstruction for the Silurian showing the locations of Laurentia, Gondwana, Oaxaquia and the Rheic Ocean after Keppie and Ramos (1999).……………………………………………………………………………… 67
21: Paleocontinental reconstruction for the Late Devonian (~390 Ma) showing the locations of Laurentia, Gondwana, Oaxaquia and the Rheic Ocean after Keppie and Ramos (1999)…………………………………...……….…………………….. 68
12
List of Plates
Plate Page
1: Field photograph of interbedded psammite (in light grey) and pelite (in dark grey) from the Cosoltepec Formation on the Aconcingo stream, 3.5 km NW of Coatlacco…………………………………………………………………………... 28
2: Field photograph of quartzite of the Coatlacco unit at the village of Coatlacco on the Coatlacco river. Picture shows the massive appearance of quartzite and S2 trending NE-SW……………..…………………………………………………….. 30
3: Field photograph shows pillow structures rimmed with red chert in dark green metabasalt of the Coatlacco unit at the village of Coatlacco on the Coatlacco river. A red chert bed approximately a meter thick is interbedded with the lava flows…………………..………………………………………………………….... 30
4: Field photograph of pink felsic intrusion in dark grey quartzites of the Coatlacco unit 2.5 km upstream from the village of Coatlacco on the Coatlacco river. Intrusion is approximately half a meter wide………………………………………………………………………………... 31
5: Field photograph of sinistral shear zone in amphibolite of the Piaxtla Suite of along Aconcingo stream 0.75 km east of Aconcingo. Quartz-rich bands define a prominent foliation………………………………………………………………… 32
6: Field photograph of FCF1 fold in quartz vein (yellow line) with axial planar SCF1 cleavage (red line) in psammites of the Cosoltepec Formation taken along Aconcingo stream, 0.25 km south of Aconcingo…………..……………………… 34
7: Field photograph of FCF2 fold in quartz vein with SCF1 and SCF2 in psammites of the Cosoltepec Formation taken along Aconcingo stream 0.75 km SW of Aconcingo Red line shows SCF1 fabric and green line shows axial planar SCF2 fabric……………………………………………………………………………….. 35
8: Field photograph of megascopic FCF3 fold (yellow line) in psammites of the Cosoltepec Formation of the Coatlacco area along Aconcingo stream, 0.75 km W- SSW of Aconcingo………………………………………………………………… 35
9: Photomicrograph of psammite (sample AMW 3A; xpl, x10 magnification, field of view 4mm) from the Cosoltepec Formation taken along Aconcingo stream, 0.25 km SE of Aconcingo. Photograph shows SCF1 as a greenschist facies fabric defined by muscovite, SCF2 as a spaced crenulation cleavage with significant 13 solution, and SCF3 forming a crenulation cleavage, crenulating both SCF1 and SCF2, again with solution (yellow lines)…………………………………………………. 36
10: Field photograph shows an FCF2 fold in quartz vein (red line) refolded by FCF3 (blue line) in dark grey pelites of the Cosoltepec Formation on the Coatlacco River, 1km south of Coatlacco…………………………………………………….. 38
11: Field photograph of FCF4 kink bands (yellow line) folding a composite SCF1/SCF2 foliation in pelites of the Cosoltepec Formation on the Coatlacco River 1 km south of Coatlacco………………………………………………………… 38 12: Photomicrograph of pelite (sample CM 5; ppl, x 10 magnification, field of view 4mm) from the Cosoltepec Formation from Coatlacco River 1 km south of Coatlacco. Photograph shows a prominent SCF1 parallel lamination defined by iron staining. SCF1 forms a slaty cleavage, whereas SCF2 forms a weak obliquely cross-cutting crenulation cleavage (blue lines)…………………………………. 39 13: Photomicrograph of greenschist facies metabasite (sample AMW 2; xpl, x 10 magnification, field of view 4mm) from the Cosoltepec Formation 0.25 km south of Aconcingo on the Aconcingo stream. Photograph shows SCF1 (yellow line) folded by FCF2 fold (red line) in a quartz vein producing SCF2. FCF2 and SCF2 have subsequently been folded by FCF3 (green line)………………………………….. 42 14: Field photograph of SCU1 and SCU2 in quartzite of the Coatlacco unit at the village of Coatlacco on the Coatlacco River. SCU1 forms a solution cleavage while SCU2 is a spaced crenulation cleavage…………………………………………… 43
15: Photomicrograph of quartzite (sample CM2; xpl, x 10 magnification, field of view 4mm) from the Coatlacco unit at the village of Coatlacco on Coatlacco River. Photograph is dominated by detrital quartz with a fine grained quartz-rich matrix. SCU1/SCU2 fabric trends E-W defined by very fine aligned muscovite mica………………………………………………………………………………. 44 16: Photomicrograph of Coatlacco basalt (sample CB 5; xpl, x 10 magnifications, field of view 4mm) from the Coatlacco unit taken from lava flow at the village of Coatlacco, on Coatlacco River. Photograph shows fresh clinopyroxene (augite- diopside) in a fine-grained altered groundmass…………………………………… 45
17: Photomicrograph of amphibolite of the Piaxtla Suite Coatlacco area (sample BG 1; xpl, x 4 magnification, field of view 4mm) from Aconcingo stream 1.5 km SE of Aconcingo. Photograph shows partially relict actinolite dominating the sample……………………….……………………………………………………. 49
14 18: Photomicrograph of granitoid (sample BG 6; xpl, x 4 magnification, field of view 4mm) of the Piaxtla Suite from Aconcingo stream 1.5 km SE of Aconcingo. Photograph shows folded foliation……………………………………………….. 50
15 INTRODUCTION
The Acatlán Complex of southern Mexico’s Mixteco terrane (Fig. 1) constitutes the largest inlier of Paleozoic rocks in the country. Tectonically juxtaposed against
Mesoproterozoic (ca. 1.0 Ga) basement, upon which are deposited Early Paleozoic platform strata of Gondwana affinity (Robison and Pantoja-Alor, 1968), the Acatlán
Complex has been interpreted as a vestige of the Iapetus suture (Ortega-Gutiérrez et al.,
1999). However, recent geochronological studies (Keppie et al., 2004a; Talavera-
Mendoza, 2005; Keppie et al., 2006) suggest that the complex is younger than the Iapetus
Ocean, the closure of which, in North America, records the accretion of peri-Gondwanan terranes to Laurentia in the Late Ordovician – Silurian. Instead, the data support its association with the Rheic Ocean (Keppie & Ramos, 1999) whose closure occurred with the collision of eastern Laurentia and northern Gondwana during the late Paleozoic assembly of Pangea. Critical constraints for this scenario can be provided by the deformational history of the Acatlán Complex, the depositional age of its sedimentary successions, and the geochemical affinity of the mafic volcanic rocks with which some of these successions are related.
This study focuses on a previously unmapped area of the Acatlán Complex, which exposes rocks that have traditionally been correlated with the structurally lowest unit of the Acatlán Complex, the siliciclastic Cosoltepec Formation (Ortega-Gutiérrez et al.,
1999) and contains important units of locally pillowed mafic volcanics. A combined structural, geochemical and geochronological examination of the metasedimentary and metaigneous assemblages was performed, in an attempt to demonstrate the relationship of 16 the rocks in this area to the Cosoltepec Formation, and in turn the linkage of this unit to the Rheic Ocean.
GEOLOGY OF MEXICO
The geodynamic evolution of Mexico has resulted from the interaction of five tectonic plates – North American, Pacific, Rivera, Cocos and Caribbean – with much of mainland Mexico falling within the boundaries of the North American plate (Ortega-
Gutiérrez et al., 1994). Sedlock et al. (1993) subdivided Mexico into sixteen terranes: two of which are of Laurentian provenance, seven of Gondwana provenance, and a further seven of Pacific provenance. Of particular interest to this study is the Mixteca terrane of southern Mexico.
Mixteca terrane and the Acatlán Complex
Mixteca terrane
The Mixteca terrane (Fig. 1) is located in southern Mexico and consists of the
Acatlán Complex, which forms its basement, and an overlying sequence of Mesozoic and
Cenozoic strata. On its eastern side, the Acatlán Complex is separated from the ca.1 Ga
Oaxacan Complex, which forms the basement of the Oaxacan (Zapoteco) terrane, by the north-south dextral Caltepec Fault zone of Permian age (Elias-Herrera and Ortega-
Gutiérrez, 2002). To the south, the Mixteca terrane is juxtaposed against the Chatino terrane by the east-west dextral Chacalapa Fault (Ducea et al., 2004). To the west, the terrane is over thrust by the Guerrero terrane along the west-dipping Papalula Thrust. In 17 the north it is overlain by Mesozoic-Cenozoic rocks and the Trans-Mexican Volcanic
Belt (Yañez et al., 1991; Ortega-Gutiérrez et al., 1999).
Error!
Figure1. Tectonostratigraphic setting of the Acatlán Complex, Mixteca terrane, southern Mexico. Guerrero, Cuicatecco and Chatino terranes are Mesozoic whereas the Oaxacan and Maya terranes have Grenville-age (ca. 1.0 Ga) basement. CF = Caltepec Fault, CAF = Chacalapa Fault, PT = Papalula Thrust (modified after Ortega-Gutiérrez et al., 1999 and Keppie et al., 2004b).
Stratigraphy
The Acatlán Complex (Figs. 2 and 3), which constitutes the basement of the
Mixteca terrane (Fig. 2), tectonically juxtaposes two main Paleozoic assemblages
(Ortega-Gutiérrez et al., 1999): the predominantly low-grade siliciclastic Cosoltepec
Formation, and the high-grade metasediments, local eclogitic mafic-ultramafic rocks and
the K-feldspar megacrystic granitoids of the Piaxtla Suite (Keppie et al., 2006). Both
assemblages are unconformably overlain by arc-related volcanosedimentary rocks of the 18 Tecomate Formation (Ortega-Gutiérrez, 1975) that Keppie et al. (2004b) have shown to be of early Permian depositional age like the lithologically similar Patlanoaya Formation.
The metapsammitic Chazumba Formation and Magdalena Migmatite, formerly thought to underlie the Cosoltepec Formation and included with the Cosoltepec Formation in the
Petlalcingo Group (Ortega-Gutiérrez, 1975; Ortega-Gutiérrez et al., 1999), have instead been shown to be Permo-Triassic units (Keppie et al., 2005; Talavera-Mendosa et al.,
2005) locally migmatized during a Jurassic thermal event (Powell et al., 1999; Keppie et al., 2004b).
Until recently the Acatlán Complex has been inferred to have undergone the following sequence of events: (i) deposition of the Cambro-Ordovician Petlalcingo Group and the oceanic Piaxtla Group, (ii) eclogite facies metamorphism, polyphase deformation and emplacement of the Piaxtla Group over the greenschist facies Petlalcingo Group during the Late Ordovician-Early Silurian Acatecan Orogeny, (iii) deposition of the
Tecomate Formation during the Devonian, and (iv) deformation and greenschist facies metamorphism during the Late Devonian Mixtecan Orogeny (Ortega-Gutiérrez et al.,
1999: Sanchez-Zavala et al., 2000).
However geochronological studies (Keppie et al., 2005; 2006; Talavera-Mendosa et al., 2005) have necessitated a revision of this sequence. In particular: (i) the Magdalena and Chazumba formations of the Petlalcingo Group have been shown to be younger than
303 Ma and 239 Ma, respectively, (ii) deposition of the Cosoltepec Formation, which is unconformably overlain by Late Devonian sedimentary rocks (Vachard and Flores de
Dios, 2002) has been shown to be younger than ca. 455 Ma (Keppie et al., 2005), (iii) the 19 tation of Ortega-Gutiérrez et al. (1999) of Nance et al. (2006). Complex comparing the traditional interpre the traditional comparing Complex atigraphic interpretation Figure 2. Tectonostratigraphy of the Acatlán with the revised tectonostr 20 Tecomate Formation has been shown to have been deposited some time between the Late
Carboniferous and Middle Permian (Keppie et al., 2004a), and (iv) the Xayacatlán
Formation in its type area at Xayacatlán has yielded an Early Silurian crystallization age
(Keppie et al., 2004a) and a continental tholeiitic geochemical signature (Dostal et al.,
2003). The major tectonothermal events have been dated as follows: (i) eclogite facies metamorphism (Acatecan Orogeny) during the Mississippian (Keppie et al., 2004), (ii) greenschist facies metamorphism (Mixteca Orogeny) during the Permo-Triassic (Malone et al., 2002; Keppie et al., 2004a), and (iii) a Jurassic episode of migmatization and high temperature / low pressure metamorphism (Keppie et al., 2004b). The revised tectonostratigraphy of the Acatlán Complex resulting from this new data is compared with that of Ortega Gutiérrez et al. (1999) in Fig. 2.
Cosoltepec Formation
The Cosoltepec Formation constitutes over 90% of the exposed Acatlán Complex and is made up of unfossiliferous quartzites, phyllites and mafic volcanics that have been repeatedly folded. The formation has been variously interpreted to represent a trench / fore arc complex (Ortega-Gutiérrez et al., 1999) or a continental rise prism (Keppie,
2004). Detrital zircon studies have yielded maximum depositional ages that range from
Late Ordovician (455 Ma; Keppie et al., 2005) to Late Devonian (410 Ma; Talavera-
Mendoza et al., 2005) and may indicate that the formation brackets a significant span of time. The formation also contains packages of basalt, studies of which have shown that they are affected by greenschist facies metamorphism and that geochemically they are 21 mainly MORB tholeiites with a minor group of oceanic island basalts and andesites
(Espinoza, 2001; Keppie et al., in press.).
East of the town of Acatlán, Malone et al. (2002) have shown that three phases of penetrative deformation affect the Cosoltepec Formation: (i) D1, which produced a
bedding-parallel cleavage that is thought to be axial planar to tight to isoclinal folds, (ii)
D2, which produced tight to isoclinal NW-NE trending curvilinear folds that deform the
first cleavage and possess an axial planar slaty cleavage, and (iii) D3, which produced
NW-SE trending inclined to upright open folds with a strong axial planar crenulation
cleavage. Malone et al. (2002) were able to demonstrate that the principle phases of
deformation in the Cosoltepec Formation (D2 and D3) are no older than Early Permian
and record north-south dextral transpression and south-vergent thrusting, and upright
north-south folding, respectively.
Petlalcingo Suite
On the basis of detrital zircon data that suggest a wide depositional age, Keppie et
al. (2006) suggested that the Cosoltepec Formation be excluded from the Petlalcingo
Group, which they have renamed the Petlalcingo Suite to include only the Chazumba
Formation and the protolith to the Magdalena Migmatite.
Chazumba Formation: The Chazumba Formation consists of amphibolite facies
metasediments dominated by quartz-biotite schists with locally present garnet, staurolite
and sillimanite. Mafic and ultramafic lenses are tectonically included with the
metasediments (Keppie et al., 2004b). The base of the Chazumba Formation is thought to 22 be transitional with the underlying Magdalena Migmatite (Ortega-Gutiérrez et al., 1999).
The youngest detrital zircon from this formation has a concordant U-Pb age of 239 ± 4
Ma (Keppie et al., 2006), constraining its maximum depositional age to the Middle
Triassic.
Magdalena Migmatite: The Magdalena Migmatite consists of polyphase deformed
metasedimentary schists, amphibolites, migmatites, calcsilicates and marbles. The type
locality of the Magdalena protolith, south of the village of Magdalena, has yielded a
concordant detrital zircon with an age of 303 ± 6 Ma (Keppie et al., 2006), showing that
its deposition can be no older than latest Carboniferous. Migmatization of this unit
occurred over a short time span during the Middle Jurassic (U – Pb ca. 175 Ma, Keppie et
al., 2004b).
Tecomate and Patlanoaya Formations
The Tecomate and Patlanoaya formations consist of pelitic and psammitic rocks,
marbles and conglomerates, which have been strongly to mildly deformed during low
grade metamorphism (Sanchez-Zavala et al., 2004). The Tecomate Formation records
two phases of penetrative deformation – N-S dextral transpression and N-S upright
folding – in contrast to the three penetrative phases recorded in the Cosoltepec Formation
(Malone et al., 2002). Zircon ages from granitoid pebbles in a conglomerate horizon of
the Tecomate Formation have yielded concordant ages of ca. 264-320 Ma (Keppie et al.,
2006) providing maximum constraints on its depositional age. Deposition has been 23 further constrained to a Pennsylvanian-Middle Permian interval by the presence of conodonts (Keppie et al., 2004c).
Piaxtla Suite
The Piaxtla Suite consists of amphibolitic to locally eclogitic mafic and ultramafic rocks, high-grade (amphibolite facies) metasediments (formally named the Xayacatlán
Formation; Ortega-Gutiérrez. et al 1999) and deformed megacrystic granitoids, which include the Esperanza Granitoids (480-440 Ma, Sanchez-Zavala et al., 2004, Talavera-
Mendoza et al., 2005, Miller et al., in press; Middleton et al., in press). Eclogite facies metamorphism of the Asis Lithodeme, a high-grade metasedimentary unit of the Piaxtla
Suite containing deformed eclogitic amphibolites, has been dated at 346 ± 3 Ma (U-Pb zircon, Middleton et al., in press). This was synchronous with migmatization at ca. 350-
330 Ma (SHRIMP, Middleton et al., in press) that is thought to be the product of decompression melting. The difference in age between the Xayacatlán Formation and the
Asis Lithodeme suggests that they cannot be correlated (Murphy et al., in press). The
Piaxtla Suite has sheared contacts with the surrounding rocks (Cosoltepec Formation,
Chazumba Formation and Magdalena Migmatite), which are of lower grade. The Suite likely contains a number of thrust slices, and has been inferred to represent obducted continental and/or oceanic lithosphere (Ortega-Gutiérrez et al., 1999). A minimum depositional age for the Piaxtla Suite is given by the Late Devonian Otate Formation, which locally overlies the suite unconformably (Murphy et al., in press).
24 GEOLOGY OF THE COATLACCO AREA
This study focuses on a previously unmapped area of the Acatlán Complex around the village of Coatlacco, east of the town of Olinalá in the state of Guerrero (Figs.
3 and 4). The rocks here are dominated by psammites and pelites, with basalts present locally, and have been correlated with the Cosoltepec Formation (Ortega-Gutiérrez et al.,
1999; Keppie, pers. comm), the lowest structural unit of the Acatlán Complex. The geochemical affinities of the basalts have the potential to provide important constraints on the depositional setting of the Cosoltepec Formation. However, it is first essential to confirm the correlation of the rocks at Coatlacco with those of the Cosoltepec Formation by assessing their deformational history, grade of metamorphism and timing of deposition.
Field relations
An area of ca. 30 km2 was mapped around the village of Coatlacco (Fig. 4) to
determine the extent and nature of the lithologies, their deformational history and their
relationship to one another. On the basis of lithology, two metasedimentary units have
been identified: (i) an interbedded pelitic – psammitic unit, and (ii) a quartzite unit with
an associated bimodal magmatic suite. Based on the deformational history the pelitic-
psammitic unit can be correlated with the Cosoltepec Formation (discussed below),
whereas the quartzite unit, which contains the basalts, is less deformed and is here named
the Coatlacco unit. Also present within the area are a group of high grade metasediments
and granitoids, and unmetamorphosed volcaniclastic rocks, sandstones and
conglomerates. Exposure is confined mainly to road cuts and stream sections. 25
Figure 3. Geological map of the northern half of the Acatlán Complex modified after Ortega-Gutiérrez et al. (1999). Map shows main lithological units along with the study area of Coatlacco east of the town of Olinalá.
26
Figure 4a. Geological map of the Coatlacco area, Acatlán Complex, Guerrero State, southern Mexico. Field map is presented in Appendix B. 27
Figure 4b. Key to field map of the Coatlacco area and location map for the field area with the main town of Cualac marked for reference.
28 Cosoltepec Formation
The Cosoltepec Formation is dominated by psammites and pelites (Plate 1).
Psammitic lithologies are typically dominant in the north of the field area and tend to be more massive in nature, whereas the pelites are laminated and dominate towards the south. The contact between the two lithologies appears to be gradational with bedding present locally; the thickness of the beds is difficult to determine due to deformation and limited exposure. Graphitic layers are locally interbedded with the psammites and pelites.
In hand specimen, quartz and mica dominate with quartz veins and quartz clasts present locally.
Plate 1. Field photograph of interbedded psammite (in light grey) and pelite (in dark grey) from the Cosoltepec Formation on the Aconcingo stream, 3.5 km NW of Coatlacco. 29 Contacts with the surrounding lithologies are not observed directly, however, the primary relationship between the pelites and psammites and their surrounding lithologies is thought to be either unconformable or faulted. Local faulting is also present bringing the psammites and pelites into contact with quartzite and basalt. Towards the north and east of the field area, unmetamorphosed sandstones, conglomerates and volcaniclastic rocks unconformably overlie the psammites and pelites.
Coatlacco unit
The Coatlacco unit comprises quartzite (Plate 2), locally pillowed metabasalts
(Plate 3) and felsic rocks that occur only in the south of the area (Fig. 4). A single felsic intrusion (Plate 4), roughly half a meter wide is present in quartzites approximately a quarter of a mile upstream from the basalts at Coatlacco. The metabasalts are typically green in outcrop and locally exhibit pillow structures defined by chert-rich rims; chert beds are also present locally within these flows (Plate 3). Calcite veins although present in the basalts at Coatlacco, are more abundant at La Cueva. In outcrop the metabasalts lack deformational structures, although they have been metamorphosed to greenstones.
30
Plate 2. Field photograph of quartzite of the Coatlacco unit taken at the village of Coatlacco on the Coatlacco River. Picture shows massive appearance of quartzite and S2 trending NE-SW.
Plate 3. Field photograph shows pillow structures rimmed with red chert in dark green metabasalt of the Coatlacco unit at the village of Coatlacco on the Coatlacco River. A red chert bed approximately a meter thick is interbedded with the lava flows.
31
Plate 4. Field photograph of pink felsic intrusion in dark grey quartzites of the Coatlacco unit 2.5 km upstream from the village of Coatlacco on the Coatlacco River. Intrusion is approximately half a meter wide.
Piaxtla Suite
Exposures of amphibolite interleaved with deformed granites are present in road
cuts and river sections along Barranca Aquixtla (Fig. 4) and have been correlated,
respectively, with the high-grade metasediments and megacrystic granitoids of the Piaxtla
Suite. The amphibolites are green in outcrop and possess a strong foliation defined by quartz-rich bands (Plate 5). The granites are both pink and white in outcrop, possess a
penetrative fabric and are locally mylonitized and folded. 32 The contact of the Piaxtla Suite with the psammites and pelites within the field area is unclear, but is inferred to be tectonic due to the variable grade of metamorphism and differing deformational history between the two units.
Plate 5. Field photograph shows sinistral shear zone in amphibolite of the Piaxtla Suite along Aconcingo stream, 0.75 km east of Aconcingo. Quartz-rich bands define a prominent foliation.
Cualac Conglomerates
To the north of Coatlacco around the town of Cualac, numerous exposures of
unmetamorphosed sandstone and conglomerate are present (Westermann et al., 1984),
which are inferred to have an unconformable contact with the surrounding
metasediments. The sandstone is typically pink and quartz-rich, whereas the
conglomerate contains quartz pebbles of variable size cemented by silica. The
conglomerate occurs at higher elevations, forming mountain-top ridges, with sandstone
occurring below in road and stream sections.
33 Undeformed Volcanic Rocks
Volcanic tuffs are present throughout the area and rest unconformably on the surrounding metasediments, infilling paleo-valleys. Their contact with the unmetamorphosed sandstones and conglomerates mentioned above is unclear but is presumed to be unconformable. The volcanic rocks are pink in color and consist of grains of variable size, shape and composition, including felsic lithologies and quartz clasts, within a fine-grained ash matrix. These tuffs have been correlated with Tertiary volcanics
(Keppie pers. comm.) associated with the subduction of the Coscos plate along the
Middle America trench (Ducea et al., 2004).
Summary
Of principle interest to this study are the Cosoltepec Formation and the Coatlacco unit. Around Coatlacco the metasedimentary rocks show multiple phases of deformation.
This is especially true towards the north of the field area where the deformation is more intense in pelitic and psammitic lithologies. Towards the south, around the village of
Coatlacco, deformation is less intense in the quartzites, which preserve detrital quartz.
The basalts, although metamorphosed and retrogressed, show little in the way of deformational fabrics and have stratigraphic contacts with the quartzites and so form part of the Coatlacco unit.
The relationship between the Coatlacco unit and the Cosoltepec Formation is exposed at a single location (Fig. 4). At La Cueva, the quartzites are in faulted contact with the psammites and pelites of the Cosoltepec Formation. However, the primary 34 relationship between the two units is thought to be that of an unconformity. Such a relationship is supported by detrital zircon data (see below).
STRUCTURE AND PETROGRAPHY
Cosoltepec Formation
Psammites
Within the psammites there are as many as three phases of penetrative deformation. In the field D1 is observed as tight to isoclinal folds (FCF1) in quartz veins
producing an axial planar solution cleavage (SCF1) (Plate 6). D2 also resulted in isoclinal
folds in quartz veins, but folds S1 to produce a second axial planar spaced crenulation
cleavage (SCF2) (Plate 7). D3 produced open to close folds with a highly variable plunge
and an associated axial planar crenulation cleavage (Plate 8).
Plate 6. Field photograph shows FCF1 fold in quartz vein (yellow line) with axial planar SCF1 cleavage (red line) in psammites of the Cosoltepec Formation taken along Aconcingo stream, 0.25 km south of Aconcingo. 35
Plate 7. Field photograph shows FCF2 fold (yellow line) in quartz vein in psammites of the Cosoltepec Formation of the Coatlacco area taken along Aconcingo stream, 0.75 km SW of Aconcingo. Red line shows SCF1 fabric and green line shows axial planar SCF2 fabric.
Plate 8. Field photograph shows megascopic FCF3 fold (yellow line) in psammites of the Cosoltepec Formation of the Coatlacco area taken along Aconcingo stream, 0.75 km W- SSW of Aconcingo.
36 In thin section the psammites (AM 1 & AMW 1, 2, 3A & 3B) are essentially composed of quartz with variable amounts of feldspar, muscovite, chlorite and calcite.
Accessory minerals include zircon and tourmaline. Metamorphic quartz typically shows a granoblastic texture and quartz veins are observed locally. The presence of metamorphic quartz suggests that muscovite and chlorite were formed under metamorphic conditions although original detrital mineralogy may be present. In thin section three fabrics are present (Plate 9). SCF1 is defined by the alignment of muscovite and locally chlorite
S2
S3 S1
Plate 9. Photomicrograph of psammite (sample AMW 3A; xpl, x10 magnification, field of view 4mm) from the Cosoltepec Formation taken along Aconcingo stream, 0.25 km SE of Aconcingo. Photograph shows SCF1 as a greenschist facies fabric defined by muscovite, SCF2 as a spaced crenulation cleavage with significant solution, and SCF3 forming a crenulation cleavage, crenulating both SCF1 and SCF2, again with solution (yellow lines).
forming a greenschist facies fabric. This has subsequently been folded to produce a
spaced SCF2 crenulation cleavage, which is enhanced by solution. FCF2 folds are evident in 37 thin section with SCF1 wrapping around the fold hinges and an axial planar cleavage
(SCF2) again defined by aligned muscovite and chlorite. SCF3 is present as a crenulation
cleavage, again with significant solution, crenulating both SCF1 and SCF2. The fabrics and
fold axes trend in a E/NE – W/SW direction (Figs. 5 & 6).
Pelites
Four phases of deformation are observed in the pelitic lithologies of the
Cosoltepec Formation. D1 and D2 are similar in nature to D1 and D2 in the psammites;
both producing isoclinal folds in quartz veins, with an associated axial planar cleavage
and spaced crenulation cleavage respectively. D3 however, resulted in smaller scale ‘Z’
and ‘S’ type folds and refolded FCF2 folds (Plate 10). A fourth phase of deformation (D4)
produced small chevron and kink folds (F4) (Plate 11), which fold a composite SCF1/SCF2 cleavage to produce kink bands.
In thin section the pelitic rocks (LCM 1 & CM 5) are composed of very fine quartz and muscovite. In sample LCM 1, micritic carbonate is dominant, replacing the matrix. Veins are rare, but where present are composed of secondary quartz and calcite with a drusy texture. In sample CM 5, a prominent SCF1 parallel lamination is defined by
iron staining and is believed to reflect original bedding (SCF0) (Plate 12). In both samples two foliations are present; SCF1 is parallel to the laminations forming a slaty cleavage,
with SCF2 trending obliquely to SCF1 as a weak crenulation cleavage (Plate 12).
38
Plate 10. Field photograph shows an FCF2 fold in quartz vein (red line) refolded by FCF3 (blue line) in dark grey pelites of the Cosoltepec Formation on the Coatlacco River, 1 km south of Coatlacco.
Plate 11. Field photograph shows FCF4 kink bands (yellow line) folding a composite SCF1/SCF2 foliation in pelites of the Cosoltepec Formation on the Coatlacco River 1 km south of Coatlacco.
39
S1
S2
Plate 12. Photomicrograph of pelite (sample CM 5; ppl, x 10 magnification, field of view 4mm) from the Cosoltepec Formation taken from Coatlacco River, 1 km south of Coatlacco. Photograph shows a prominent SCF1 parallel lamination defined by iron staining. SCF1 forms a slaty cleavage, whereas SCF2 forms a weak obliquely cross-cutting crenulation cleavage (blue lines).
Stereographic plots of the structural data for both the psammites and the pelites
show that the fold axes (FCF1-FCF4) plunge broadly NE or SW with SCF1, SCF2 and SCF3
dipping relatively steeply to the NW and SE (Figs. 5, 6 & 7). 40
Figure 5. Equal area stereographic projection of contoured poles to SCF1, SCF2 and SCF3 in the Cosoltepec Formation of the Coatlacco area, Acatlán Complex, Guerrero State, southern Mexico. Fabrics are generally relatively steeply dipping to the NW and SE, trending NE-SW. SCF1, n=104; SCF2, n=160; SCF3, n=6
Figure 6. Equal area stereographic projection showing the orientation of FCF1, FCF2, FCF3 and FCF4 fold axes in psammites and pelite of the Cosoltepec Formation of the Coatlacco area, Acatlán Complex, Guerrero State, southern Mexico. FCF1, FCF2 and FCF4 axes generally trend NE-SW, while FCF3 is more variable in its plunge.
41
Figure 7. Equal area stereographic projection of contoured poles to SCF1, SCF2 and SCF3 and their relationship to FCF1, FCF2, FCF3 and FCF4 axes of psammites and pelites in the Cosoltepec Formation of the Coatlacco area, Acatlán Complex, Guerrero State, southern Mexico. SCF1, SCF2 and SCF3 are axial planar to FCF1, FCF2, FCF3 and FCF4. SCF1, n=104; SCF2, n=160; SCF3, n=6.
Metabasic rocks
A single sample (AMW 2) is dominated by actinolite with abundant quartz, and
chlorite and epidote are locally present. The association of actinolite, epidote, chlorite
and quartz is indicative of greenschist facies metamorphism. Quartz locally shows a
granoblastic texture with tremolite and chlorite forming fine laths, which define both an
SCF1 and SCF2 fabric. The sample shows three phases of penetrative deformation. An FCF2 fold folds a pre-existing SCF1 fabric to produce SCF2, which has subsequently been refolded by FCF3 (Plate 13).
42
F2
S2 S1 F3
Plate 13. Photomicrograph of greenschist facies metabasite (sample AMW2; xpl, x10 magnification, field of view 4mm) from the Cosoltepec Formation, 0.25 km south of Aconcingo on the Aconcingo stream. Photograph shows SCF1 (yellow line) folded by FCF2 fold (red line) in a quartz vein producing SCF2. FCF2 and SCF2 have subsequently been folded by FCF3 (green line).
Coatlacco Unit
Quartzites
The quartzites (LCM 2, CM 1 & 2) of the Coatlacco unit are relatively
undeformed with only two fabrics present: (i) an SCU1 solution cleavage, and (ii) an SCU2 crenulation cleavage (Plate 14). Stereographic plots show that both cleavages dip steeply to the south-east (Fig. 8). Folding is not evident in these quartzites although locally east- west trending extensional quartz veins are present.
43
S1 S2
Plate 14. Field photograph shows SCU1 and SCU2 in quartzite of the Coatlacco unit at the village of Coatlacco on the Coatlacco River. SCU1 forms a solution cleavage while SCU2 is a spaced crenulation cleavage.
Figure 8. Stereographic plot of poles to SCU1 and SCU2 foliations in quartzites of the Coatlacco unit, Acatlán Complex, Guerrero State, southern Mexico. Fabrics are relatively steeply dipping to the SE.
44 The quartzites of the Coatlacco unit (CM 2 & LCM 2) are made up almost entirely of quartz and have undergone less intense deformation than the psammites and pelites correlated with the Cosoltepec Formation. In thin section an original clastic texture can be observed with detrital quartz grains dominating (Plate 15). Grains are cemented by cryptocrystalline quartz and quartz overgrowths are present locally. In thin section two fabrics are present, both weakly defined by aligned muscovite and to a lesser extent quartz (Plate 15), but their generation is more easily determined in hand specimen.
SCU1/SCU2 fabric
Plate 15. Photomicrograph of quartzite (sample CM2; xpl, x10 magnification, field of view 4mm) from the Coatlacco unit taken at the village of Coatlacco on Coatlacco River. Photograph is dominated by detrital quartz with a fine grained quartz-rich matrix. SCU1/SCU2 fabric trends E-W defined by very fine aligned muscovite mica.
Mafic lithologies
Basalt outcrops were sampled in two localities, Coatlacco (CB 1, CB 3 & CB 5)
and La Cueva (LCB 1, LCB 2, LCB 3 & LCB 5). The basalts at both localities occur as pillowed and massive flows. At La Cueva they are interbedded with the surrounding 45 quartzites and possess both unconformable and tectonic contacts with the psammites and pelites of the Cosoltepec Formation. Despite lacking obvious deformational structures in the field, the basalts are highly altered and retrogressed. The mineralogy of the basalts includes relict clinopyroxene (Plate 16) with variable amounts of quartz, plagioclase, epidote, calcite and chlorite. Accessory minerals include sphene. Clinopyroxene
(diopside and/or augite) typically occurs as phenocrysts, however, locally a subophitic texture is present with altered plagioclase. Clinopyroxene is typically retrogressed to chlorite. Plagioclase crystals are too small and altered for their composition to be determined. The matrix is altered and retrogressed and is composed of calcite, chlorite, actinolite and cryptocrystalline quartz. Secondary quartz and calcite are present in veins
Plate 16. Photomicrograph of Coatlacco basalt (sample CB5; xpl, x10 magnification, field of view 4mm) from the Coatlacco unit taken from lava flow at the village of Coatlacco, on Coatlacco River. Photograph shows fresh clinopyroxene (augite-diopside) in a fine-grained altered groundmass.
46 and epidote is common in one sample (LCB 1) as small porphyroblasts, occurring within the matrix and also in veins. Significant iron oxide is present in the matrix of most samples. In thin section, the chert-rimmed pillows are opaque, the chert having been altered to iron oxide. The presence of actinolite associated with chlorite and epidote indicates that the samples underwent greenschist facies metamorphism.
Comparison with the Cosoltepec Formation Type Section
In its type locality near the town of Acatlán, three phases of penetrative deformation have been observed in the Cosoltepec Formation (Malone et al., 2002): (i)
D1, which produced a bedding-parallel schistosity (S1) axial planar to isoclinal folds, (ii)
D2, which produced isoclinal sheath folds (F2) in quartz veins with an axial planar
cleavage (S2), and (iii) D3, which produced upright to inclined, open and close folds (F3) with a variable plunge to the northwest and southeast.
Mineralogically in its type area (Ortega-Gutiérrez, 1978 and Malone, 2001), the rocks of the Cosoltepec Formation are dominated by quartz with varying amounts of biotite, muscovite, chlorite, phengite, plagioclase feldspar, opaque minerals and tourmaline. Quartz typically shows a granoblastic texture and bedding sub-parallel quartz veins are abundant. The groundmass consists of fine recrystallized quartz. Garnet and staurolite have been observed locally (Ortega-Gutiérrez et al., 1999) typically towards the eastern part of the complex where metamorphism is interpreted to have reached higher grades. A strong foliation is present in all lithologies typically along bedding planes
(Malone, 2001). 47 Metavolcanic rocks are locally present in the Cosoltepec Formation, occurring as tectonic slices of massive and pillowed flows within the metasediments (Espinoza, 2001).
The volcanic rocks are typically strongly altered and penetratively deformed, but igneous textures can be easily recognized. Quenched textures are present at the margins of pillows, while doleritic and porphyritic textures are confined to the center of pillows
(Espinoza, 2001).
Around Coatlacco the first three fabrics (DCF1-DCF3) observed in the psammites
and pelites correlate with those in the Cosoltepec Formation Type Section: (i) DCF1, which resulted in tight to isoclinal folds (FCF1) in quartz veins with an axial planar
solution cleavage (SCF1), (ii) DCF2 also resulted in isoclinal folds in quartz veins, but folds
SCF1 to produce a second axial planar spaced crenulation cleavage (SCF2), and (iii) DCF3, which produced open to close folds with a highly variable plunge and an associated axial planar crenulation cleavage in coarser grained psammites and smaller scale ‘Z’ and ‘S’ type folds and refolded FCF2 folds in pelitic lithologies. A fourth phase of deformation –
DCF4, which produced small chevron and kink folds folding a composite SCF1/SCF2
cleavage to produce kink bands – is present in the psammites and pelites of the Coatlacco
area. The deformational phases present in the psammites and pelites are difficult to
correlate with those present in the quartzites since the quartzites lack folding. However,
SCU1 and SCU2 in the Coatlacco unit are sub-parallel to the composite SCF1/SCF2 foliation
and SCF3 in the Cosoltepec Formation, and SCU2 in the Coatlacco unit, like SCF3 in the
Cosoltepec Formation, is a crenulation cleavage.
Around Coatlacco, the pelites and psammites have also undergone greenschist facies metamorphism. Muscovite typically dominates in the samples with chlorite 48 abundant in a single sample (sample AMW 1) and present in variable amounts in other samples. A single sample (sample AMW2) is dominated by actinolite and epidote. The consistent presence of key index minerals in some samples indicates variable degrees of greenschist facies metamorphism, which has not reached the grade observed towards the east of the Acatlán Complex (Ortega-Gutiérrez et al., 1999). The quartzites are dominated by detrital quartz with overgrowths present locally indicating a lower grade of metamorphism. The metabasalts, although metamorphosed and retrogressed, remain undeformed and original igneous textures are not present, unlike in the type Cosoltepec
Formation.
On the basis of structure and petrography the psammites and pelites of the
Coatlacco area can be correlated with the Cosoltepec Formation. On the basis of differing deformational history and mineralogy the quartzites and metabasalts are not assigned to the Cosoltepec Formation and are here named the Coatlacco unit.
Piaxtla Suite
A tectonic slice of amphibolites and granitoids (Piaxtla Suite on Fig. 3) occurs towards the north of the area that can be correlated with the Piaxtla Suite.
Amphibolites
The amphibolites (BG 1 & 3A/B) consist essentially of amphibole
(cummingtonite) that occurs as coarse-grained porphyroblasts as well as finer laths and needles (Plate 17). Variable amounts of epidote and albite are also present along with rare calcite. These rocks have been metamorphosed to a higher grade (amphibolite facies) 49 than the metasediments and metabasalts of the Cosoltepec Formation and the Coatlacco unit.
Plate 17. Photomicrograph of amphibolite of the Piaxtla Suite Coatlacco area (sample BG1; xpl, x 4 magnification, field of view 4mm) from Aconcingo stream, 1.5 km SE of Aconcingo. Photograph shows partially relict actinolite dominating the sample.
It is uncertain whether the amphibolite can be correlated with the high grade
metasediments of the Xayacatlán Formation, which, in the type area, has yielded an
earliest Silurian age (442 ± 1 Ma; Keppie et al., 2004a) or with those of the Asis
Lithodeme (346 ± 3 Ma; U-Pb zircon age, Middleton et al., in press), which are thought
to be intruded by granitoids dated at 460 Ma (Miller et al., 2002).
Granitoids
Granitoids (BG 4, 6 & 7) are dominated by quartz and feldspar (perthite,
plagioclase and anorthoclase) with variable amounts of calcite, chlorite, muscovite and 50 accessory minerals. Typically poly- and mono- crystalline quartz is layered with laths of muscovite, which are locally retrogressed to chlorite. Feldspar typically occurs as coarser relict phenocrysts. Calcite occurs as a secondary mineral phase in veins, along with secondary quartz, which is present in variable amounts in all samples. The granitoid rocks are typically massive although locally microscopic F2 folds (Plate 18) and
mylonitic bands are defined by stretched out grains of quartz, however this is better
observed in hand specimen. It is possible to correlate these granitoids with megacrystic
granitoids seen elsewhere in the Acatlán Complex (480-440 Ma, Sanchez-Zavala et al.,
2004, Talavera-Mendoza et al., 2005, Miller et al., in press; Middleton et al., in press).
S1
Plate 18. Photomicrograph of granitoid (sample BG6; xpl, x4 magnification, field of view 4mm) of the Piaxtla Suite from Aconcingo stream, 1.5 km SE of Aconcingo. Photograph shows folded foliation (yellow line).
51 GEOCHEMISTRY
Eight samples were taken for major and trace element geochemical analysis from two localities near the village of Coatlacco and at La Cueva (Appendix C). Major and trace elements (Table 1) were analyzed using X-ray fluorescence (fused glass discs and powder pellets) at the Geochemical Centre of Saint Mary’s University, Halifax, Nova
Scotia. The precision and accuracy of this technique are discussed in Dostal et al. (1986,
1994).
Whole – rock geochemistry
The samples show a range in major element abundances. They are sub-alkaline basalt to andesitic in composition (Fig. 9) with silica content (SiO2) ranging from 44.32 to 50.84 wt%. Concentrations of other elements are presented in Table 1. A single sample
(CB 5) shows significantly greater concentrations of TiO2, Fe2O3 and P2O5, and lower
abundances of Na2O and Al2O3 compared to the other samples.
Trace element concentrations in all samples are more variable, but sample CB 5
again shows significant deviations from the trend. For example, Cr in all samples ranges
from 172 to 333 ppm, but is only 46 ppm in sample CB5. Similar trends can also be seen
in some of the other trace elements (Table 1). The Coatlacco and La Cueva basalts range
from 24 – 300 ppm Cr and 28 – 143 ppm Ni, suggesting that the basalts range from
primitive and less fractionated to highly evolved and fractionated (Fig. 10), reflecting
varying degrees of pyroxene fractionation. The variations in concentration of mobile
elements such as Nb, Rb, Pb, U, Sr and Ba likely reflect variable mobility during
metamorphism and alteration. 52
Figure 9. Zr/TiO2 vs. SiO2 discrimination diagram of basalts from this study (in pink) and those of Keppie et al. (in press) (in blue). Basalts plot generally as sub-alkaline basalts with a single sample from this study and two samples from Keppie et al. (in press) plotting as andesites.
Cr vs N
300
250
200 Increasing 150 fractionation
100 This study
50 Keppie et al (in press) 0 0 100 200 300 400 500 Cr (ppm
Figure 10. Plot of Cr vs. Ni shows variable degrees of fractionation in basalts from this study (pink) and Keppie et al. (in press) (blue). Arrow shows the direction of increasing fractionation. 53
Interpretation
Interpretation of the geochemical data is based primarily on immobile elements, such as the high field strength elements (HFSE) Ti, Y, Zr and Nb, Th, and transition elements such as Ni and Cr. The low field strength elements (LFSE), such as Na, Rb, Pb and U, are easily mobilized during metamorphism and alteration and are thus not useful for identifying primary magmas. Sr and Ba, although less mobile, are not necessarily indicative of the original magma composition, and should be interpreted with caution.
The major elements are generally considered mobile during metamorphism and alteration and are generally not useful in identifying the source of the primary magma.
The results are plotted with data from Keppie et al. (in press) (Table 2) taken from samples within the Cosoltepec Formation throughout the Acatlán Complex for comparison. A Zr/TiO2-SiO2 discrimination diagram (Fig. 9) indicates both sample sets
are of sub-alkaline to andesitic composition. On a Ti-Zr-Y ternary diagram (Fig. 11), the
basalts plot in the within-plate field, and on a plot of Cr versus Ti (Fig. 12), the samples
are low potassium tholeiites or within-plate basalts. On Miyashiro plots (Figs. 13 and 14)
the samples again plot in the tholeiite field. Tholeiitic basalts are characterized as having
higher degrees of FeO enrichment with increasing FeO/MgO ratio (Fig. 14), which is a
measure of advancing fractional crystallization, than the calc-alkalic series (Miyashiro,
1974). Miyashiro (1974) also states that basalts and basaltic andesites of the tholeiitic
series are characteristic of immature island arcs.
54
Figure 11. Ti-Zr-Y discrimination diagram for basalts from this study (pink) and from those of Keppie et al. (in press) (blue) (after Pearce and Cann, 1973). Samples plot as within-plate basalts with a single sample from this study plotting in the MORB – island- arc – calc-alkali field.
Cr vs T 100000 This study Ocean Floor Basalts Keppie et a 10000 (in press)
1000
Low Potassium Tholeiites 100 10 100 1000 Cr (ppm
Figure 12. Plot of Cr vs. Ti for basalts from this study (pink) and from those of Keppie et al. (in press) (blue). Basalts plot as low potassium tholeiites. 55 FeO/MgO vs SiO 70.00 Calc- 60.00 alkalic 50.00 40.00 30.00 This study 20.00 Tholeiitic 10.00 Keppie et a (in press) 0.00 0 5 10 15 20 FeO/MgO wt%
Figure 13. Plot of FeO/MgO vs. SiO2 (after Miyashiro, 1974) places basalts on the boundary between calc-alkalic and tholeiitic compositions with a single sample from this study (pink) and two samples from Keppie et al. (in press) (blue) plotting as tholeiitic basalts.
FeO/MgO vs Fe 40 35 30 Tholeiitic 25 20 15 ` This study 10 Calc-alkali 5 Keppie et al (in press) 0 0 5 10 15 20 FeO/MgO wt%
Figure 14. Plot of FeO/MgO vs. FeO (after Miyashiro, 1974) places samples from this study (pink) and those from Keppie et al. (in press) as tholeiitic basalts.
56 The data have been plotted on a primitive mantle normalized spider diagram
(Fig.15) with average trends for MORBs, ocean island basalts (OIBs) and continental basalts. The data of Keppie et al. (in press) is shown for comparison. The basalts from this study show enrichments in Th, La and Nd relative to the other elements along with significant Nb and Ti anomalies. The mobile elements, Rb and Ba, show a wide variation in concentration due to their mobility and so should be interpreted with caution. Thorium, although a LFSE, which are generally mobile, behaves more like an HFSE and can be regarded as being immobile during metamorphism and alteration, its concentration reflecting the primary magma (Kean et al., 1995). This wide variation in Th consequently indicates a highly variable magma composition. The Nb and Ti anomalies along with the general jagged appearance of the spider diagram shown by the basalts of this study are indicative of crustal or arc contamination. The relatively high Y indicates a deep source for the magma; Y behaves like an HREE and, in this case, is relatively low compared to the LREE suggesting that the magma was derived from a deeper garnet lherzolite mantle than a shallower spinel lherzolite. This is also consistent with a partial crustal component, particularly indicative of continental rifting. Continental rift zones are also characterized by enriched concentrations of Na2O (Brownlow, 1996), which is
evident in some of the samples.
The basalts can thus be interpreted as sub-alkali to tholeiitic basalts erupted in a
continental rift setting. The basalts at Coatlacco exhibit pillow structures rimmed with
chert and chert beds within these lava flows, indicating a sub-aqueous eruptive setting.
57
Figure 15. Primitive mantle normalized abundance patterns for basalts from the Coatlacco and La Cueva areas plotted with average element concentrations for MORBs, OIBs and continental basalts (after Rollinson, 1994). Colored lines are data from this study while yellow polygon shows the range of data of Keppie et al. (in press).
Comparison with the Cosoltepec Formation Type Section
In the type Cosoltepec Formation basaltic slices have been observed throughout the Acatlán Complex. Recent geochemical studies (Espinoza, 2001; Keppie et al., in press) indicate these basalts are of oceanic composition (Figs. 9-15). Keppie et al. (in press) interpret their basalts as mainly MORB tholeiites (Fig. 15), with a minor group of oceanic island basalts and andesites interpreted to have been incorporated into the metasediments of the Cosoltepec Formation during a late Fammenian – Mississippian 58 deformational event (Keppie et al., in press). This is in contrast to the basalts at Coatlacco and La Cueva, which have continental geochemical signatures. This allows the basalts at
Coatlacco and La Cueva and those of the type Cosoltepec Formation to be interpreted as different magmatic suites erupted in different tectonic settings. This is further supported by their stratigraphic contacts with surrounding quartzites, which allows the basalts to be included in the Coatlacco unit.
GEOCHRONOLOGY
Two metasedimentary units (LCB3 & LCM3; Tables 3 & 4; Appendix C) were sampled for detrital zircons and U-Pb geochronology was performed at the Department of
Geosciences, University of Arizona. One hundred zircons were analyzed from each sample with cores of grains being preferred to avoid dating possible metamorphic rims.
Zircons were analyzed in polished section with a Micromass Isoprobe multicollector
ICPMS equipped with nine Faraday collectors, an axial Daly collector and four ion- counting channels (Dickinson and Gehrels, 2003). The Isoprobe is equipped with an ArF
Excimer laser ablation system with an emission wavelength of 193 nm. The configuration of the collector allows 204Pb to be measured in an ion-counting channel, while 206Pb,
207Pb, 208Pb, 232Th and 238U are measured simultaneously with Faraday detectors. The
analysis was conducted in static mode with a laser beam diameter of 50 microns,
operated with an output energy of ~32 mJ (at 23 kV) and a pulse rate of 8 Hz. Each
analysis consisted of one twenty-second integrations on peaks with no laser firing and
twenty one-second integrations on peaks with laser firing. Hg contribution to the 204Pb 59 mass position was removed by subtracting on-peak background values. Inter-element fractionation was monitored by analyzing an in-house zircon standard, which has a concordant TIMS age of 564 ± 4 Ma (2σ) (Gehrels, unpublished data). This standard was analyzed once for every five unknown grains. U and Th concentrations were monitored by analyzing a standard (NIST 610 Glass) with ~500 ppm Th and U. The lead isotopic ratios were corrected for common Pb using the measured 204Pb, assuming an initial Pb
composition according to Stacey and Kramers (1975) and respective uncertainties of 1.0,
0.3 and 2.0 for 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb.
The systematic error is defined by the grouping of the age of the standard,
calibration correction from the standard, composition of common Pb and the uncertainty
of the decay constant. For these samples the systematic errors are ~1.2-1.3% for
206Pb/238U and ~0.8-1.0% for 206Pb/207Pb. The age probability plots (Ludwig, 2003) used
in this study were constructed using the 206Pb/238U age for young (<1.3 Ga) zircons and
the 206Pb/207Pb age for older (>1.3 Ga) grains. In old grains, ages with >20% discordance
or >5% reverse discordance are considered unreliable and were not used. Analyses with
an error greater than 10% were also rejected. Each significant age peak was dated by
selecting the best ages and their associated errors for the zircons comprising each age peak and calculating their weighted average using Isoplot (Ludwig, 2003). This gives an average age for each peak with an associated error (Figs. 16 & 17). The error was then recalculated using the following formula: ((√ (measured error)2 + (systematic error)2) /
100) * the age of the peak. Measured error is the error calculated by isoplot, whereas systematic error is associated with the analyses. This calculation is in accordance with 60 Gehrels (unpublished data) to give a more accurate representation of the error associated with the age of each peak.
Cosoltepec Formation
Sample LCM 3 was taken in the pelitic-psammitic unit, which has been correlated with the Cosoltepec Formation. The spectrum shows a range in zircon ages, but the youngest concordant detrital zircon in sample LCM3 yields a 206Pb/238U age of 462 ± 15
Ma (Middle Ordovician) (Fig. 16). There is a significant cluster of concordant zircon
ages at 459 ± 14 Ma, which are likely derived directly from plutons within the Acatlán
Complex, such as the Esperanza Granitoids (480 - 440 Ma; Sanchez-Zavala et al., 2004;
Talavera-Mendoza et al., 2005; Miller et al., in press; Fig. 19). The sample is dominated
by zircons ranging in age from ca. 900 Ma to ca.1250 Ma (Fig. 16). These zircons require
a Grenville-age provenance, for example, eastern Laurentia or, more likely, the
neighboring Oaxaquia microcontinent (Miller et al., in press; Fig. 18). The older zircons
with ages in the range ca. 1300-1500 Ma are of uncertain origin but may imply cratonic
sources in Laurentia, Baltica, Amazonia or West Africa.
Coatlacco unit
Sample LCB3 was taken in the Coatlacco unit from quartzites that are interbedded
with the basalts. Since these rocks are in stratigraphic contact the maximum depositional
age of the sediments can also be taken as the maximum age for the extrusion of the
basalts. The Coatlacco unit has a significantly wider age spectrum than the Cosoltepec
Formation. The youngest concordant detrital zircon yields a 238U/206Pb age of 386 ± 7 Ma 61 (Middle Devonian) (Fig. 17), the provenance of which is likely the Piaxtla Suite, the high-grade metamorphism of which is Late-Devonian – Mississippian (Middleton et al., in press; Murphy et al., in press).
Figure 16. Cumulative probability plot and Concordia diagram (inset) for psammites (sample LCM3) from the Cosoltepec Formation of the Coatlacco area taken along Aconcingo stream west.
A significant cluster of zircon ages occurs at 563 ± 22 Ma, possible provenances for which include the peri-Gondwana terranes, the Brasiliano belt of Brasil, the Yucatan
Peninsula (Keppie et al., 2006) and the Pan-African belts of West Africa (Fig. 18). Two other significant peaks occur at ca. 837 ± 28 Ma and ca. 1156 ± 74 Ma. The ca.1 Ga zircons are likely derived directly from Oaxaquia, while the Neoproterozoic zircons are likely derived from eastern Amazonia, for example the Goias magmatic arc (Pimental et al., 2000; Figs. 19 and 21). The zircons in the age range ca. 1400 – 2150 Ma have 62 possible provenances in Amazonia and the West African Craton. The Paleoprotozoic to
Archean zircons (ca. 2440 – 2592 Ma) are again likely derived from Gondwana, with a single possible source in Central Amazonia (Fig. 18).
Figure 17. Cumulative probability plot and Concordia diagram (inset) for quartzite (sample LCB3) from the Coatlacco unit taken along Coatlacco River at La Cueva.
Comparison with the Cosoltepec Formation Type Section
The depositional age of the Cosoltepec Formation has been constrained to the interval Late Ordovician – Carboniferous by U-Pb dating of detrital zircons (youngest concordant detrital zircon: 455 ± 4 Ma; Keppie et al., 2006) and the maximum age of the unconformably overlying Tecomate Formation, which may be as old as Late
Pennsylvanian (Keppie et al., 2004c). On the basis of structure and lithology, the pelitic- 63 psammitic unit at Coatlacco can be correlated with the Cosoltepec Formation. This unit
(sample LCM 3) has a youngest concordant detrital zircon age of 462 ± 15 Ma, similar to the youngest concordant detrital zircon in the type Cosoltepec Formation (455 ± 4 Ma;
Keppie et al., 2006). However, the age spectrum of the type Cosoltepec Formation is different from that of the Cosoltepec Formation at Coatlacco, although the range of detrital zircons present in the Cosoltepec Formation at Coatlacco lies within that of the type Cosoltepec Formation. This suggests different contributions from crustal components of the same source area. The ages present in the Coatlacco unit, on the other hand, are significantly different from those observed in both the type Cosoltepec
Formation and the Cosoltepec Formation at Coatlacco. Notably, the age range of detrital zircons in the Coatlacco unit is far wider than those seen in units elsewhere in the Acatlán
Complex, indicating distinct sediment sources (Fig. 18).
Interpretation
The maximum depositional ages and the age spectrums for the Cosoltepec
Formation (youngest concordant detrital zircon 462 ± 15 Ma) and the Coatlacco unit
(youngest concordant detrital zircon 386 ± 7 Ma) suggest changes in source areas between ca. 460 Ma and ca. 385 Ma. The Cosoltepec Formation is dominated by ages in the range ca. 900 – 1250 Ma (Fig. 16) with a population of young zircons at 459 ± 14 Ma.
The young zircons are likely derived from plutons within the Acatlán Complex itself, for instance the Esperanza Granitoids, which have yielded U-Pb ages in the range ca. 480 –
440 Ma (Sanchez-Zavala et al., 2004; Talavera-Mendoza et al., 2005; Miller et al., in press; Middleton et al., in press). This is in agreement with Keppie et al. (2006) who 64 interpret the provenance of their ca. 473 Ma zircons in the same way. However, zircons in the range 900 – 1250 Ma require a Grenville-age provenance, such as eastern
Laurentia or Oaxaquia. According to the continental reconstruction proposed by Ortega-
Gutiérrez et al. (1999), Oaxaquia and the eastern margin of Laurentia were juxtaposed
Figure 18. Detrital zircon age populations in the Acatlán and Oaxacan Complexes compared to age provenances in eastern Laurentia, Baltica, west Avalonia, Amazon Craton and the west African Craton. Colored bars show age populations for this study and that of the type Cosoltepec Formation. Xayacatlán and Tecomate populations are those of Talavera-Mendoza et al. (2005) (after Nance et al., 2006 and Murphy et al., in press).
by the end of the Ordovician (Fig. 19). However, Fig. 18 shows that there is a wide age
range of zircon provenances in eastern Laurentia, which are not seen in the Cosoltepec
Formation at Coatlacco. If the zircons were derived from Laurentia a contribution from 65 other age provenances, such as the Yavapai-Mazatzal orogenic belt (1.6 – 1.8 Ga;
Karlstrom and Humphreys, 1998; Fig. 18), must be expected. Such ages are present, for example, in Ordovician foreland sediments of the Laurentian margin (Cawood and
Nemchin, 2001). That these ages are not present suggests that Oaxaquia was not proximal to Laurentia at this time. This supports the continental reconstruction of Keppie and
Ramos (1999), which places Oaxaquia on the eastern margin of Gondwana during the
Middle Ordovician, at considerable distance from Laurentia (Fig. 20).
Figure 19. Paleocontinental reconstruction for Late Ordovician – Early Silurian after Ortega-Gutiérrez et al. (1999). Model envisages the Acatlán Complex to be part of the Iapetus suture formed by the docking of Oaxaquia against the southern margin of Laurentia.
The Coatlacco unit is dominated by zircons with ages in the range 500 – 700 Ma
with significant peaks at ca. 350 Ma, ca. 800 - 900 Ma and ca. 1.1 – 1.2 Ga (Fig. 18). The
Coatlacco unit also has a wide range of older zircons in the range ca. 1600 – 2150 Ma
with an additional peak at ca. 2250 Ma. The age spectrum of the Coatlacco unit is 66 significantly wider than that of the Cosoltepec Formation at Coatlacco and the type
Cosoltepec Formation (Fig. 18), indicating different source areas and a different paleogeographic position for the Acatlán Complex during the Middle Devonian. A similar peak of Cambrian to Neoproterozoic zircons seen in the type Cosoltepec
Formation is interpreted by Keppie et al. (2006), to record provenances in the Yucatan
Peninsula or the Brasiliano orogens of South America. A similar interpretation can be made for the Cambrian – Neoproterozoic zircons in the Coatlacco unit in accord with the interpretation that Oaxaquia bordered the western margin of South America in the
Silurian – Late Devonian (Keppie et al., 2006; Fig. 21). The Grenville-age zircons are consistent with a source in Oaxaquia, however, Laurentia and Baltica could also be possible sources. The ca. 800 – 900 Ma zircons are most likely derived from Amazonia since a gap of this age occurs in the age provinces of Laurentia and Baltica (Fig. 18).
This further supports the position of Oaxaquia on the eastern margin of Gondwana.
Zircons with ages in the range ca. 1600 – 2150 Ma require cratonic provenances in
Laurentia, Baltica or Amazonia. The oldest zircons in the Coatlacco unit are likely derived from Central Amazonia. The wide age range of zircons in the Coatlacco unit suggests a variety of sources for these sediments including eastern Laurentia, Baltica,
Amazonia and West Africa.
Various models have been proposed for the position of the Acatlán Complex during the Ordovician - Devonian. Ortega-Gutiérrez et al. (1999) interpreted the complex to be part of the Iapetus suture, formed as a result of the collision of the Oaxaquia microcontinent with the southeastern margin of Laurentia in the Late Ordovician-Early
Silurian (Fig. 19). Alternatively, Keppie and Ramos (1999) suggest Oaxaquia formed part 67 of the Columbia margin of Gondwana and remained attached to Gondwana until the closure of the Rheic Ocean (Figs. 20 and 21). Evidence from the Piaxtla Suite suggests that Rheic Ocean closure occurred in the Late Devonian – Mississippian (Middleton et al., in press). If so the prominent Neoproterozoic peak in detrital zircon ages could reflect the approach of the Acatlán Complex to east Laurentia as the Rheic Ocean closed and the emergence of previously accreted peri-Gondwana terranes as a sediment source. Hence, the data support the model of Keppie and Ramos (1999) but suggest by the late Middle
Devonian Gondwana and Laurentia were in proximity to each other than their reconstructions indicate and that the Rheic Ocean was approaching closure (Fig. 20 and
21).
Figure 20. Paleocontinental reconstruction for the Silurian showing the locations of Laurentia, Gondwana, Oaxaquia and the Rheic Ocean after Keppie and Ramos (1999). 68
Figure 21. Paleocontinental reconstruction for the Late Devonian (~390 Ma) showing the locations of Laurentia, Gondwana, Oaxaquia and the Rheic Ocean after Keppie and Ramos (1999).
DISCUSSION
On the basis of lithology, structure and age, two metasedimentary rock units have been identified in the vicinity of Coatlacco: (i) a pelitic-psammitic unit, which can be
correlated with the Cosoltepec Formation, and (ii) a quartzite and basaltic unit with minor
felsic intrusions, here named the Coatlacco unit.
The Cosoltepec Formation comprises pelites and psammites that record four
phases of deformation: (i) isoclinal folds (FCF1) in quartz veins with an associated
greenschist axial planar cleavage (SCF1), (ii) isoclinal folds (FCF2) again in quartz veins,
but also folding SCF1 to produce a second axial planar spaced crenulation cleavage (SCF2),
(iii) open to closed and ‘Z’ and ‘S’ type folds (FCF3) on a scale of a few centimeters to a 69 few meters with an associated crenulation cleavage (SCF3), and (iv) chevron and kink
folds (FCF4), which fold a composite SCF1/SCF2 foliation. These phases of deformation
correlate with those observed in the type locality of the Cosoltepec Formation. On the
basis of this it is possible to correlate the psammites and pelites at Coatlacco with the
type locality of the Cosoltepec Formation west of the town of Acatlán. This is further
supported by detrital zircon dating of these sediments, which yield a youngest concordant
238U/206Pb age of 462 ± 15 Ma that is consistent with ages from the Cosoltepec Formation
elsewhere (455 ± 4 Ma; Keppie et al., 2006).
The Coatlacco unit consists of quartzites that have undergone two phases of mild
deformation: SCU1, which resulted in a solution cleavage, and SCU2, which produced a
crenulation cleavage. Folding is not evident in these rocks. The rocks have undergone
low-grade metamorphism (greenschist facies) shown by the presence of aligned
muscovite but preserve an original sedimentary texture. The basalts, with which the
quartzites are locally interbedded, are essentially undeformed and preserve primary
clinopyroxene and have undergone greenschist facies metamorphism indicated by the
presence of chlorite, actinolite and epidote. It is possible to correlate SCU1 and SCU2 in the
Coatlacco unit with SCF2 and SCF3 in the Cosoltepec Formation.
The basalts exposed around Coatlacco are interbedded with the quartzites and so
are assigned to the Coatlacco unit. The basalts have geochemical signatures consistent
with a within-plate tectonic setting and are interpreted to record an extensional rifting
event. This is further supported by a felsic intrusion within the quartzites that may
indicate bimodal volcanism during tectonism, if these are of the same age. The presence
of pillows and chert within the lava flows indicates a sub-aqueous setting. However, data 70 from Keppie et al. (in press) and Espinoza (2001) show oceanic geochemical signatures in the basalts of the Cosoltepec Formation, indicating that the basalts of the Coatlacco area represent a different magmatic suite than those in the Cosoltepec Formation.
Detrital zircon ages from a quartzite of the Coatlacco unit show this extensional event to be no older than 386 ± 7 Ma (youngest concordant detrital zircon). Hence, the rifting event recorded by the Coatlacco unit cannot be linked to the opening of the Rheic
Ocean, which occurred at ca. 500 – 475 Ma (Keppie and Ramos, 1999). Instead, the basalts must record a younger extensional event. The maximum age for this event
(Middle to Upper Devonian) is consistent with the timing of the exhumation of the
Piaxtla Suite during the Late Devonian-Mississippian (eclogite facies metamorphism dated at 346 ± 3 Ma), the P-T-t path for which indicates rapid exhumation in an extensional setting (Middleton et al., in press). Rapid exhumation like that recorded by the Piaxtla Suite is generally related to continent-continent collision, for example, rapid exhumation due to subduction during continental collision (Ernst, 1988), exhumation due to lateral thrust ramps (Hynes and Eaton, 1999), and extensional exhumation in areas of transtension during continent-continent collision (Reddy et al., 2002). The exact mechanism for the exhumation of the Piaxtla Suite is unclear, however, a combination of the models by Ernst (1988) and Hynes and Eaton (1999) is preferred (Middleton et al., in press). The geochemistry of the basalts and the timing of their extrusion suggest that it is this extensional event they are recording.
A link can also be made between the Coatlacco unit and the Tecomate Formation.
The Tecomate Formation consists of greenschist facies conglomerates, sandstones, slates and volcaniclastic units of basalt – andesite composition (Sánchez Zavala et al., 2000). 71 The age of the Tecomate Formation is constrained by the presence of latest
Pennsylvanian – Middle Permian conodonts (Keppie et al., 2004a) and, like the
Coatlacco unit, the formation records two sets of structures: (i) isoclinal folding, and (ii) upright open folding with an axial planar crenulation cleavage. Since these structures correlate with DCF2 and DCF3 in the Cosoltepec Formation they also correlate with DCU1 and DCU2 in the Coatlacco unit. However, the youngest concordant detrital zircon age
from the Coatlacco unit (386 ± 7 Ma), is significantly older than the youngest detrital
zircon (SHRIMP ages ca. 264-320 Ma Keppie et al., 2006) in the Tecomate Formation
and the oldest provenances for the Coatlacco unit (ca. 1400 – 2150 Ma and ca. 2440 –
2592 Ma) are significantly older than the range of detrital zircons in the Tecomate
Formation (Fig. 19). Hence, this correlation is not favored.
The detrital age spectrum of the Coatlacco unit is similar to those of the
Magdalena Migmatite and Chazumba Formation (Keppie et al., 2006), in that all three
units are dominated by Mesoproterozoic sources. The Chazumba Formation consists of
metapsammites and metapelites with tectonic lenses of mafic-ultramafic rocks, which
underwent amphibolite facies metamorphism during the Jurassic (Keppie et al., 2004b).
The Chazumba Formation grades downwards into a similar unit that also contains
calcsilicate and marble lithologies, which were locally deformed and migmatized in the
Jurassic to produce the Magdalena Migmatite (Keppie et al., 2004b). Since this grade of
metamorphism and these lithologies are not observed in the Coatlacco unit, a correlation
with the Chazumba Formation and Magdalena Migmatite is also not favored.
The maximum depositional age for the Cosoltepec Formation is constrained to the
Middle Ordovician (youngest concordant detrital zircon 462 ± 15 Ma), whereas the 72 Coatlacco unit was deposited no earlier than the upper Middle Devonian (youngest concordant detrital zircon 386 ± 7 Ma). Both of these ages are consistent with deposition within the margin of the Rheic Ocean. The data from both samples support the model proposed by Keppie and Ramos (1999), that is, that Oaxaquia formed part of the
Columbia margin of Gondwana and remained attached to Gondwana until the closure of the Rheic Ocean in the Late Devonian – Mississippian (Middleton et al., in press).
Detrital zircon ages in the Cosoltepec Formation allow the Acatlán Complex to be placed on the northern margin of Oaxaquia, bordering Gondwana in the Middle Ordovician.
Zircon ages and provenances in the Coatlacco unit are consistent with the continued position of the Acatlán Complex on the eastern margin of Gondwana but on approaching the eastern margin of Laurentia during the closure of the Rheic Ocean. This model is further supported by the extensional exhumation of the Piaxtla Suite, which the Coatlacco unit is interpreted to record. Hence, the data support the model of Keppie and Ramos
(1999) but suggest that the Rheic Ocean was close to closure by the late Middle
Devonian.
CONCLUSIONS
This study focused on a previously unmapped area of the Acatlán Complex, around the village of Coatlacco, Guerrero State, southern Mexico. The purpose of the study was to carry out geologic mapping, structural, geochemical and geochronological analysis of the metasediments and their associated basalts to assess the correlation of these rocks to the Cosoltepec Formation and their linkage to the Rheic Ocean. 73 The findings of this study can be summarized as follows. On the basis of age, structure and lithology two metasedimentary rock units have been identified in the
Coatlacco area: (i) a pelitic-psammitic unit, which records up to four phases of deformation and can be correlated with the Cosoltepec Formation, and (ii) a quartzite unit with associated basalts and felsic intrusions, which record only two phases of deformation and is here termed the Coatlacco unit. In the Cosoltepec Formation the structural history is as follows: (i) isoclinal folds (FCF1) in quartz veins with an associated
greenschist axial planar cleavage, (ii) isoclinal folds (FCF2) again in quartz veins, but also
folding SCF1 to produce a second axial planar spaced crenulation cleavage (SCF2), (iii)
open to close and ‘Z’ and ‘S’ type folds (FCF3) on a scale of a few centimeters to a few
meters with an associated crenulation cleavage (SCF3), and (iv) chevron and kink folds
(FCF4), which fold a composite SCF1/SCF2 foliation. The structural history of the Coatlacco
unit is only preserved in the quartzites: (i) SCU1, which resulted in a solution cleavage, and (ii) SCU2, which produced a crenulation cleavage. Folding is not evident in these
rocks. Both the Cosoltepec Formation and the Coatlacco unit have undergone greenschist
facies metamorphism.
Geochemical studies of the basalts within the Coatlacco unit indicate they are
subalkali – tholeiitic within-plate continental basalts recording an extensional event. U-
Pb dates for detrital zircons from the quartzites with which the basalts are interbedded
indicates the basalts record a post – mid – Devonian extensional event (386 ± 7 Ma) and
not the opening of the Rheic Ocean. This extensional event is consistent with the
exhumation of the Piaxtla Suite (Late Devonian-Mississippian; eclogite facies metamorphism 346 ± 3 Ma; Middleton et al., in press). 74 U-Pb dates for detrital zircons from the Cosoltepec Formation (youngest concordant detrital zircon 462 ± 15 Ma) are consistent with the maximum depositional age of the type Cosoltepec Formation (455 ± 4 Ma; Keppie et al., 2006). The Coatlacco unit is significantly younger than the Cosoltepec Formation although the age of both units is consistent with deposition in the Rheic Ocean. The maximum depositional ages and provenances for both metasedimentary units support the model of Keppie and Ramos
(1999) that the Acatlán Complex was on the eastern margin of Oaxaquia, bordering northern Gondwana in the Middle Ordovician and remained there until its approach to eastern Laurentia during the closure of the Rheic Ocean in the Late Devonian.
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81 Appendix A. Thin section descriptions
Sample location N17 43.341 W098 39.305
Sample location N17 43.341 W098 39.305
82
Sample location N17 42.080 W098 38.307
Sample location N17 43.233 W098 3.353
83
Sample location N17 43.233 W098 38.353
Sample location N17 42.080 W098 38.307
84
Sample location N17 42.080 W098 38.307
Sample location N17 42.080 W098 38.307
85
Sample location N17 41.484 W098 39.289
Sample location N17 41.484 W098 39.289
86
Sample location N17 43.080 W098 38.307
Sample location N17 42.080 W098 38.307
87
Sample location N17 42.080 W098 38.307
Sample location N17 41.484 W098 39.289
88
Sample location N17 41.484 W098 39.289
Sample location N17 41.484 W098 39.289
89
Sample location N17 41.484 W098 39.289
Sample location N17 43.539 W098 38.914
90
Sample location N17 43.539 W098 38.914
Sample location N17 43.539 W098 38.914
91
Sample location N17 43.539 W098 38.914
Sample location N17 43.539 W098 38.914
92 Appendix B. Field map of the Coatlacco area, Acatlán Complex, Mixteca terrane, southern Mexico
93 Key to field map of the Coatlacco area, Acatlán Complex, Mixteca terrane, southern Mexico
94 Appendix C. Geochemistry and geochronology sample location map
95 Table 1. Geochemical data for basalt samples from Coatlacco and La Cueva, Acatlán Complex, Mixteca terrane, southern Mexico
CB1 CB4 CB5 CB6 LCB5 LCB6 LCB7 LCB8 (wt %) SiO2 50.84 46.85 44.32 46.17 44.82 45.79 47.58 48.22 TiO2 1.629 1.464 2.824 1.22 1.133 1.199 1.93 1.208 Al2O3 14.07 14.01 12.12 15.91 15.15 14.49 15.65 15.36 CaO 7.62 8.95 10.56 10.54 8.12 9.96 6.05 7.61 K2O 0.16 0.02 0.21 0.05 1.36 0.06 0.28 0.79 MgO 6.62 8.68 4.47 7.88 7.7 6.8 7.69 8.5 MnO 0.298 0.267 0.294 0.166 0.294 0.169 0.174 0.168 Na2O 4.66 2.5 1.26 3.32 3.21 2.98 4.81 3.75 P2O5 0.168 0.133 0.304 0.106 0.094 0.103 0.162 0.102 FeO3 10.1 12.64 17.83 11.16 10.51 11.03 8.9 11.5 (T) TiO2 1.629 1.464 2.824 1.22 1.133 1.199 1.93 1.208 L.O.I 3.12 4.42 4.91 3.6 7.74 6.32 6.73 3.69 Sum 99.29 99.93 99.1 100.12 100.13 98.9 99.96 100.9
(ppm) Rb 6 3 2 <2 22 <2 17 12 Sr 79 79 51 55 120 88 129 65 Ba 175 214 515 234 309 223 183 284 Th 8 13 16 8 8 6 12 12 U 2 2 1 1 2 1 3 2 Y 21 16 18 14 15 17 24 14 Zr 73 66 164 59 47 50 90 53 Nb 5 8 12 4 4 7 7 7 Ni 71 74 40 107 142 105 102 118 Co 51 61 72 55 63 54 64 62 V 240 261 392 222 207 225 303 218 Cr 231 172 46 320 319 333 215 352 La 33 38 63 31 31 32 38 33 Nd 39 47 67 39 37 37 40 42 Ga 12 20 23 18 15 17 12 17 Zn 100 85 143 80 82 75 105 81 Pb 11 16 18 10 10 8 14 14
96 Table 2. Geochemical data of Keppie et al. (in press) for basalts from the Cosoltepec Formation, Acatlán Complex, Mixteca terrane, southern Mexico
COS- COS- COS- COS- COS- COS- COS- COS- COS- Sample 3 5 9 16 31 33 44 46 52 (wt %) SiO2 51.86 51.60 49.21 47.74 46.95 47.88 47.44 47.08 47.39 TiO2 2.36 2.19 1.08 1.45 1.31 1.33 1.15 1.82 1.23 Al2O3 15.01 15.64 14.74 14.49 15.12 15.62 15.52 14.09 14.88 Fe2O3 15.24 16.07 10.14 12.32 11.79 9.58 10.33 13.41 10.99 MnO 0.09 0.14 0.15 0.20 0.20 0.26 0.19 0.15 0.19 MgO 2.53 2.08 7.88 6.67 7.22 7.60 8.54 7.33 7.95 CaO 2.20 1.70 9.52 11.81 10.92 9.47 9.49 9.69 11.96 Na2O 6.24 6.95 3.88 2.29 2.75 3.51 2.08 3.08 2.53 K2O 0.24 0.25 0.26 0.22 0.69 0.62 1.67 0.56 0.22 P2O5 0.62 0.88 0.08 0.11 0.10 0.12 0.11 0.15 0.10 LOI 2.56 2.10 2.53 2.53 2.95 3.89 3.33 2.68 3.44 Total 98.94 99.60 99.47 99.84 100.00 99.89 99.85 100.04 100.87
(ppm) Cr 358 423 323 88 306 315 268 154 451 Ni 126 285 81 73 110 119 117 102 130 Co 61 100 46 47 52 54 43 63 49 V 270 253 198 255 238 243 211 299 222 Cu 44 49 62 97 120 132 57 163 107 Pb 5 5 1 5 0 0 0 3 3 Zn 90 141 94 110 65 81 68 90 47 Rb 11 12 10 7 20 22 34 16 8 Ba 5 5 118 49 117 218 608 62 133 Sr 73 84 124 165 196 93 100 215 124 Ga 15 14 11 18 17 15 16 18 15 Ta 2.45 2.34 0.08 0.13 0.23 0.30 0.41 0.40 0.26 Nb 51.7 49.8 3.4 3.9 6.3 6.5 2.7 8.7 4.7 Hf 4.34 4.04 1.63 2.22 2.02 2.00 2.18 2.85 1.73 Zr 210 193 62 83 72 74 81 108 64 Y 18 28 20 26 18 19 21 22 16 Th 5.08 4.85 0.09 0.15 0.32 0.33 0.73 0.53 0.29 U 0.00 0.00 0.00 1.00 0.50 2.00 0.50 1.00 0.50 La 16.90 26.53 1.68 2.67 3.83 4.05 4.68 5.97 3.48 Ce 43.44 62.54 5.47 7.79 10.13 10.56 11.81 15.80 9.04 Nd 22.06 25.85 5.61 7.90 7.92 8.29 8.88 12.08 7.13 Sm 4.64 5.41 2.09 2.76 2.41 2.47 2.60 3.53 2.15 Eu 1.42 1.85 0.81 1.05 0.91 0.93 0.92 1.33 0.83 Gd 4.58 5.93 3.08 4.11 3.18 3.28 3.49 4.40 2.88 Tb 0.69 0.90 0.55 0.75 0.56 0.58 0.61 0.74 0.51 Dy 4.04 5.33 3.75 5.07 3.62 3.85 4.02 4.73 3.29 Ho 0.69 0.94 0.73 1.01 0.72 0.75 0.82 0.91 0.66 Er 1.88 2.51 2.14 2.95 2.06 2.19 2.41 2.58 1.92 Tm 0.25 0.33 0.32 0.43 0.30 0.31 0.35 0.37 0.27 Yb 1.49 1.99 2.07 2.78 1.94 2.01 2.29 2.35 1.81 Lu 0.22 0.29 0.30 0.41 0.28 0.29 0.34 0.33 0.26
97 Table 3. Detrital zircon data for psammites of the Cosoltepec Formation (sample LCM 3), Coatlacco area, Acatlán Complex, Mixteca terrane, southern Mexico
98 Table 4. Detrital zircon data for quartzites of the Coatlacco unit (sample LCB3), Coatlacco area, Acatlán Complex, Mixteca terrane, southern Mexico
99 ation and quartzites of the Coatlacco unit, xteca terrane, southern Mexico xteca terrane, psammites and pelites of the Cosoltepec Form Table 5. Structural data for Coatlacco area, Acatlán Complex, Mi