Tectonic Significance of Cretaceous–Tertiary Magmatic and Structural

IN PRESS

Tectonic signiﬁ cance of Cretaceous–Tertiary magmatic and structural evolution of the northern margin of the Xolapa Complex, Tierra Colorada area, southern Mexico

L.A. Solari† R. Torres de León G. Hernández Pineda J. Solé G. Solís-Pichardo Instituto de Geología, Universidad Nacional Autónoma de México, 04510 Mexico City, Mexico T. Hernández-Treviño Instituto de Geofísica, Universidad Nacional Autónoma de México, 04510 Mexico City, Mexico

ABSTRACT volcanic rocks, and open folding during D4. have markedly different basement characteris- These four pulses of subduction-related mag- tics, including metamorphic grade, composition, The Tierra Colorada area sits along the matism in the Tierra Colorada area indicate ages, and tectonic histories. According to Campa northern limit of the Xolapa Complex, where a regular northeastward subduction at the and Coney (1983), and as modiﬁ ed by Sedlock et it is juxtaposed against the Mixteco (Paleo- Mesoamerican trench since Jurassic time, al. (1993), these terranes are: (1) the Guerrero ter- zoic) and Guerrero (Mesozoic) terranes of and alternate with contractile and/or exten- rane, mainly composed of Mesozoic arc-related southern Mexico, just north of Acapulco. sional tectonic events. The gap in magmatic rocks (Centeno-García et al., 1993; Elías-Herrera This paper presents combined structural and activity ca. 90–100 Ma roughly coincides et al., 2000); (2) the Mixteco terrane, consist- geochronological data from Tierra Colorada with deposition of platformal limestones of ing of the Paleozoic Acatlán Complex, which is area that show evidence of four deformational the Morelos Formation during the middle characterized by an assemblage of high-grade events and several episodes of arc magmatism Cretaceous. The stable conditions during oceanic and continental rocks faulted against during Mesozoic and Cenozoic time. The old- deposition of the Morelos Formation may low-grade pelites and psammites, and covered est magmatism is represented by ca. 165 Ma have resulted from a combination of back- by late Paleozoic–Jurassic continental sediments granitoids and was followed by intrusion of arc extension and development of a passive (Ortega-Gutiérrez et al., 1999; Keppie et al., the foliated El Pozuelo granite (129 ± 0.5 Ma; margin during the Early–middle Cretaceous, 2004; Talavera-Mendoza et al., 2005; Nance et concordant U-Pb zircon analysis). This which postdated the accretion of an exotic al., 2006); (3) the Oaxaca terrane, the Grenvillian intrusion postdates D1 metamorphism and block, either the Guerrero terrane or the Oaxacan Complex basement of which comprises migmatization in the Xolapa Complex. The Chortís block. Following the Laramide orog- rift-related anorthosite- mangerite- charnockite- next magmatic episode is represented by eny in southern Mexico (roughly during the granite, paragneiss, volcanic-arc rocks, and the peraluminous, foliated El Salitre granite Late Cretaceous) the Paleocene–Miocene tec- calcsilicates metamorphosed to granulite facies (55.3 ± 3.3 Ma; mineral–whole-rock Rb-Sr tonic evolution in the Tierra Colorada area (Keppie et al., 2003; Solari et al., 2003); and isochron) and the protomylonitic Las Piñas involved an alternation of magmatic pulses (4) the ~600 × 50–80 km Xolapa terrane, which I-type granite (54.2 ± 5.8 Ma; lower intercept with extensional and contractile events. This bounds the other three terranes on the south (e.g., U-Pb zircon). Las Piñas granite is character- was the result of a combination of several fac- Herrmann et al., 1994; Corona-Chávez, 1997; ized by D2 ductile fabric with normal, top-to- tors, including the geometry of the subducted Werre-Keeman and Bustos-Díaz, 2001; Ducea et the north-northwest sense of shear, deformed slab, convergence rate, stress transmission al., 2004; Corona-Chávez et al., 2006). at 45–50 Ma (Rb-Sr and K-Ar ages). The between the subducting and overlying plates, The Xolapa Complex, the basement of the ca. 34 Ma undeformed granites correspond and the rate of subduction erosion. Xolapa terrane, has been interpreted as an to the last intrusive pulse in the area, postdat- allochthonous, Jurassic–Cretaceous, deformed

ing both D3 south-southwest–verging thrust- Keywords: southern Mexico, U-Pb geochro- magmatic arc terrane (Ortega-Gutiérrez et al., ing of the Cretaceous Morelos Formation nology, Xolapa Complex, Cenozoic tectonics, 1995; Morán-Zenteno et al., 1996; Dickinson over sheared granites and Lower Cretaceous arc magmatism. and Lawton, 2001; Schaaf et al., 2002; Corona- Chávez et al., 2006), or an autochthonous mag- INTRODUCTION matic arc (Herrmann et al., 1994; Meschede et †Present address: Centro de Geociencias, Uni- versidad Nacional Autónoma de México, Campus al., 1996; Ducea et al., 2004; Keppie, 2004). Juriquilla, 76230 Querétaro, Querétaro, México; so- Southern Mexico is composed of a mosaic of The boundaries between terranes in southern [email protected]. at least four different crustal terranes (Fig. 1) that Mexico are generally marked by regional shear

GSA Bulletin; Month/Month 2007; v. 119; no. X/X; p. XXX–XXX; doi: 10.1130B26023.1; 7 ﬁ gures; 2 tables.

Mesozoic Xolapa Complex A Mexico City Oaxaca fault Paleozoic-Mesozoic (?) Mazatlán Complex 050100 km Puebla Sierra de Juarez Mylonitic Complex Permian Caltepec fault Paleozoic Acatlán Complex Cuernavaca Mid-Jurassic Precambrian Oaxacan Complex (right lateral) Laramide Precambrian Guichicovi Complex rePapalutla n fault Acatlan ?Triassic Gue r ro terra e (thrust) Major shear zones 18° At ne Zpo Maya terra oterra at ne Chilpancingo c Cu ca ec ter Laramide (thrust) La Venta shear zone Mixte terraneeco ito (Early Eocene) Studied area (Fig. 2)

XT Oaxaca Matías Romero TC rane 17° Acapulco Xolap t r ane Tertiary (thrust, Laramide 97°00' USA normal, Oligocene/Miocene) aer Tehuantepec 30° Chacalapa shear zone (Oligocene) MEX Salina Cruz 16° IC Pacific Ocean Pacif O Huatulco Gulf of Mexico ic Oc 98° Puerto Ángel 96° MEXICO CITY B ean Zihuat. Aca. Huat. 16° Fig. 1a 110° 100° 90° Figure 1. (A) Terrane map of southern Mexico (modiﬁ ed after Campa and Coney, 1983; Sedlock et al., 1993; Kep- pie, 2004). The square indicates the studied area as depicted in Figure 2. Position of the terrane map is marked by a rectangle in the inset of B. Locations in italics in B: Zihuat.—Zihuatanejo; Aca.—Acapulco; Huat.—Huatulco.

zones or faults (Fig. 1). (1) The Caltepec fault during the Early Cretaceous by the subduc- about a pole near Santiago, Chile (Keppie and zone is a Permian dextral shear zone between tion of the Mezcalera plate and closure of the Morán-Zenteno, 2005). the Mixteco and Oaxaca terranes (Elías-Herrera Arperos basin (e.g., Dickinson and Lawton, The southern Mexico structural pattern was and Ortega-Gutiérrez, 2002). (2) The Tertiary 2001, and references therein), or a continental revised by Nieto-Samaniego et al. (2006), who Laramide Papalutla fault forms the boundary fringing arc (e.g., Elías-Herrera and Ortega- proposed grouping deformation structures into between the Guerrero and Mixteco terranes Gutiérrez, 1998). The Chortís block currently three events (shortening, strike slip, and nor- (e.g., Cerca et al., 2004). (3) The Chacalapa–La constitutes the basement of the northern part of mal faulting), from the Late Cretaceous to the Venta shear system is an Eocene–Oligocene Central America. Two alternative models have Miocene, progressively younging toward the sinistral strike fault along the northern border of been proposed for the southern Mexico-Chortís east. Detailed structural observations and geo- the Xolapa Complex (Ratschbacher et al., 1991; connections. (1) The Chortís block was adja- chronology of the Tierra Colorada area bear Riller et al., 1992; Tolson, 2005; this paper). cent to southwestern Mexico until the middle on the tectonic evolution of southern Mexico. Mesozoic and Cenozoic reconstructions of Tertiary, when it started to move toward its Combining the new data presented here with southern Mexico need to be based upon the current position along a transform fault paral- those previously published (e.g., Riller et al., correct time assignment of signiﬁ cant geologic lel to the Middle America trench; this fault is 1992; Herrmann et al., 1994; Meschede et al., events, such Laramide shortening and Paleo- now represented by the Acapulco trench and 1996; Ducea et al., 2004; Nieto-Samaniego et cene–Oligocene arc magmatism (e.g., Cerca the Polochic-Motagua fault zone in Guatemala al., 2006) provides a wider database for testing et al., 2004; Morán-Zenteno et al., 2005; this (Anderson and Schmidt, 1983; Pindell et al., some of the previous models, such as the age of study), the time of migmatization in the Xolapa 1988; Ross and Scotese, 1988; Herrmann et al., migmatization, the migration of structural pat- Complex (66–46 Ma according to Herrmann 1994; Schaaf et al., 1995; Morán Zenteno et al., terns, and pulses of magmatism. et al., 1994; Jurassic–Cretaceous according to 1996; Meschede et al., 1996). (2) Keppie (2004) Ducea et al., 2004), and possible interactions also placed the Chortís block adjacent to south- LITHOLOGICAL UNITS between southern Mexico with the Guerrero ern Mexico in the Pangean reconstruction, but terrane and the Chortís block. The Guerrero by Eocene time it was southwest of its current In the fairly well exposed geologic record of terrane occupies most of western Mexico, and position. The Chortís block would have reached the Tierra Colorada area (Fig. 2), the southern- has been interpreted as either an exotic intraoce- its present position rotating clockwise, covering most unit is represented by the Xolapa Complex, anic terrane accreted to southwestern Mexico a distance of ~1100 km during the Cenozoic which is composed of high-grade orthogneiss

2 Geological Society of America Bulletin, Month/Month 2007 IN PRESS Cretaceous–Tertiary magmatic and structural evolution, Tierra Colorada area, southern Mexico Alluvial deposits Xolapa Complex Balsas Fm. Morelos Fm. El Pozuelo granite El Salitre Granite Tierra Colorada Intrusive Papagayo Fm. Las Piñas granite Chapolapa Fm. 5.3 1.8 159 144 206 180 Synclinal Thrust Normal Fault Foliation and lineation Observed geologic contact Inferred geologic contact Bedding Highway Paved road Dam River Inhabited area / town Dated samples stations Stereonet Sigma indicators, indicating S2 top-to-the NNW shearing (on section) Unpaved road Unpaved 65.0 99.0 0.01 23.8 54.8

33.7

Shear indicators, indicating (on map) S2 sheared units

L MIOCENE

OLIGOCENE EOCENE

RL PALEOCENE

D Symbols

A ID

HOLOCENE E

EARLY M PLEISTOCENE

LATE EPOCH LATE

PLIOCENE EOGENE PALEOGENE N

JURASSIC CEOUS T A

D CRE

IO TER- TERTIARY

NARY HW 95 QUA 95 ER MEX

P MESOZOIC A CENOZOIC ER 37 N 64 99° 27´ 30´´ 500 400 300 200 100 m.a.s.l. NE B ned as indicated by large arrows in the map. ned as indicated by large arrows eonets with main poles of S1 and S2 foliation, N 74 Xolapa NE 50 2km 1 S poles 21 ¿J?Gn 1km 38 400 12 200 31 El Zapote 0 35 66 19 25 99° 30´ 20 Omitlán 58 51 3 64 Villa Guerrero Villa D 69 46 73 2 La Palma 9 S Papagayo River 2 Spoles 4 D 41 68 MEX 200 43 N 2 C. ALTO DEL C. ALTO TEPEHUAJE Tierra 55 S Colorada 45 LV0321 Papagayo River, S Papagayo River, 13 4 D B C. LAS PIÑAS 2 L (335º/50º avg) 2 4 S D Las Piñas 34 95 MEX 40 44 53 58 99° 32´ 30´´ 70 Dam 51 HW 95 34 65 67 48 68 44 75 45 63 50 70 61 46 51 2 S poles 62 35 2 40 3 S N Vieja Venta 44 30 68 N Xolapa NW 60 50 3 45 52 1 60 21 S poles Highway, El Pozuelo Highway, 50 2 L (326º/65º avg) 62 47 56 65 95 MEX 42 40 99° 35´ 54 42 47 2 S poles 52 XO0201 77 73 N 65 El Papagayo 71 2 50 S poles Papagayo River, N Papagayo River, 78 51 77 N C. DEL PEREGRINO 2 Xolapa 65 A L (346º/46º avg) W of the dam, Highway 89 12 5Km A 80 LV0136 2 L (355º/34º avg) 31 SW 0 99° 37´ 30´´ 2 km west 17° 05´ 500 400 300 200 100 17° 10´ 17° 07´ 30´´ m.a.s.l. in the legend. Ster reported with stratigraphic column and section. Symbols are Colorada area, Tierra Figure 2. Geologic map of of ~2 km length, positio performed in outcrops were also shown. Structural measurements lineation, are as well L2 stretching

Geological Society of America Bulletin, Month/Month 2007 3 IN PRESS Solari et al. and paragneiss and marbles that have undergone The El Pozuelo granite crops out in the west- Structurally above the El Pozuelo, Las Piñas, variable degrees of migmatization. The gneisses ern part of the studied area (Fig. 2), southeast of and El Salitre granitoids is a 2–5-km-thick are generally fi ne to medium grained, and rarely the El Salitre granite. Its best outcrop is located green volcaniclastic unit that was originally des- reach augen gneiss texture. Representative min- along the Mexico-Acapulco toll road, where it is ignated as Chapolapa Formation (De Cserna, erals of these rocks are biotite, muscovite, and a fi ne-grained, gray-greenish, foliated body that 1965). Pelitic and psammitic metasedimen- green hornblende; however, metasedimentary grades eastward into a porphyritic facies. It is tary bands are intercalated with andesitic and bands, most Al rich, also contain sillimanite, made up of quartz, K-feldspar > plagioclase, and rhyolitic metavolcanic rocks with arc-related staurolite, and garnet. The gneissic banding biotite intergrown with green hornblende. Zir- affi nities (De Cserna et al., 1994). The Chapo- consists of <3-mm-wide domains of oriented con, apatite, and magnetite are the most common lapa Formation ranges from undeformed to biotite, muscovite, and/or hornblende alternat- accessory minerals. Hernández-Pineda (2006) mylonitic (Riller et al., 1992). Where contacts ing with bands composed of an association of reported geochemical data for El Pozuelo granite with the underlying granitoids are exposed, they quartz, K-feldspar, and/or plagioclase. The min- as La/Nb > 4, subalkaline affi nity, A/CNK index are generally affected by ductile deformation. eral lineation is defi ned by oriented biotite and <1.1 (Maniar and Piccoli, 1989); together with Undeformed metavolcanic rocks are character- hornblende crystals. absence of muscovite, the presence of biotite and ized microscopically by the presence of pla- A series of variably foliated granitoids is on green hornblende, these data indicate that it is a gioclase and quartz phenocrysts in a very fi ne top of the Xolapa gneisses (Fig. 2). The plutons calc-alkaline I-type granite (according to the ter- grained matrix of plagioclase, chlorite, and epi- occur along a west-northwest–trending band minology of Kemp and Hawkesworth, 2004). dote. Deformed portions show a complete range parallel to a complex structure that has been Foliation is penetrative toward the northern between protomylonite, mylonite, and ultramy- called the La Venta–Tierra Colorada shear zone exposure along Highway 95, where it is mainly lonite (e.g., Figs. 3C and 4B–4D). U-Pb zircon (e.g., Riller et al., 1992). According to Campa characterized by oriented biotite and stretched geochronology on these rocks yielded crystalli- and Coney (1983), this structure should mark quartz, the latter forming a penetrative stretch- zation ages of 126–133 Ma (Campa and Iriondo, the contact between the Mixteca and Xolapa ing lineation with constant north-northwest 2004; Hernández-Treviño et al., 2004). terranes. The El Salitre granite crops out along orientation (Fig. 3A). The El Pozuelo southern- Albian–Cenomanian limestones of the More- Federal Road 95 just west of Xolapa town, west most contact with the migmatites of the Xolapa los Formation (Fries, 1960; Hernández-Romano of the mapped area (Fig. 2). This is a peralumi- Complex along the highway is a brittle normal et al., 1997) are thrust over the Chapolapa For- nous body mainly made up of quartz, plagio- fault; however, its original intrusive relationship mation in the entire area (Figs. 3D–3G). The clase, K-feldspar, abundant muscovite, scarce is indicated by several dikes emanating from the limestones are unconformably overlain by con- garnet, and accessory apatite. The <5-mm-long main granitic body (e.g., Fig. 3B). tinental conglomerates of the late Paleocene– muscovite crystals defi ne the foliation, which At the structural top of the Xolapa basement early Oligocene Balsas Formation (Fries, 1960; is not a mylonitic fabric. In places the granite is a protomylonitic granite that we name Las Cerca et al., 2004; Molina-Garza and Ortega- grades into a pegmatitic facies with the same Piñas (Fig. 2); it crops out continuously east Rivera, 2006), and acid volcanic rocks of the mineralogy. Morán-Zenteno (1992) dated some of Highway 95. It is made up of plagioclase, Papagayo Formation, which is possibly the pegmatites as 59 ± 1 Ma (Rb-Sr whole-rock– quartz, K-feldspar, biotite, titanite, and second- same age as the Alquitrán Formation, 22–24 Ma muscovite isochron) near El Salitre, although ary minerals such as chlorite and epidote. Zir- (Hernández-Treviño et al., 1996). The Balsas their relationships with the main granitic body con is a common accessory phase. Petrographi- Formation was not observed in direct contact are not described in detail. The El Salitre gran- cally, it ranges between granite and granodiorite with the Xolapa Complex, as previously noted ite generally contains xenoliths of metamorphic from west to east, and it has two main variants: in other localities (e.g., De Cserna, 1965; Werre- rocks, as large as 20 cm, such as amphibolites, a southern undeformed variety and a central- Keeman and Bustos-Díaz, 2001). metagraywackes, and metapelites, likely derived northern protomylonitic granite (Fig. 4A). The large Tierra Colorada granitic to grano- from the Xolapa Complex. The westernmost Where the contacts are exposed and not affected dioritic body (34 Ma, concordant U-Pb zircon contact of the El Salitre granite, outside the by brittle faulting or ductile shearing, Las Piñas age; Herrmann et al., 1994) intrudes the Las mapped area, is intruded by the ca. 29–34 Ma granite is intrusive into the Xolapa Complex and Piñas granite, Chapolapa Formation, Morelos Xaltianguis granite (Schaaf et al., 1995; Ducea Chapolapa Formation (Fig. 2). Its geochemical Formation, and Xolapa basement. The Tierra et al., 2004), whereas toward the east it grades character is strikingly similar to the El Pozuelo Colorada granite is only affected by brittle into an unfoliated facies. granite (Hernández-Pineda, 2006). deformation, and is intruded by diabase dikes,

Figure 3. Outcrop pictures representing main structures as described in text. (A) El Pozuelo granite, in its deformed facies, with visible

S2 foliation (subvertical), and L2, almost vertical white stretched quartz ribbons. Scale is indicated by white pencil in the middle left. (B) A pegmatite emanating from the El Pozuelo granite intruding migmatitic gneisses of the Xolapa Complex. Hammer is ~35 cm long.

(C) F3 kink-like folds developed in the S2 mylonitic foliation in the Chapolapa Formation, just south of the Papagayo River dam. Top-to-the south-southwest sense of shearing is indicated by the black arrows. Scale is indicated by a ruler in the top left of the picture. Ruler is 30 cm long. (D) South-southwest–verging D3 thrust of the Morelos Formation on top of the sheared Chapolapa Formation, as observed along the Acapulco–Mexico Highway. Main thrust plane is drawn and indicated by black and white triangles. A circle in the bottom indicates the drag fold illustrated in Figure 3G. (E) Broken and thrusted chert lens into the Morelos Formation. D3 top-to-the south-southwest sense of shearing is indicated by black arrows. Coin is 25 mm in diameter. (F) Same contact represented in D, with Morelos Formation thrusted over the Chapolapa Formation, as indicated by stair-stepping faults. Man is 1.60 m tall. (G) Folded beds at the bottom of the Morelos Formation. An

F3 drag fold is developed near the contact with the underlying previously mylonitized Chapolapa Formation (bottom left). Sense of shearing is indicated by the white arrow. Notebook is 4 × 6 inches.

4 Geological Society of America Bulletin, Month/Month 2007 IN PRESS Cretaceous–Tertiary magmatic and structural evolution, Tierra Colorada area, southern Mexico

A B

SSW NNE C D

MORELOS LIMESTONES

CHAPOLAPA FM.

SSW NNE SSW Detail of Fig. 3G NNE E F

STONE E

S LIM TION

ELO

OR NNE SSW FORMA M

G MORELOS L APA IMESTO NES POL CHAPOLAPA HA C FM.

Folded S2 NNE SSW

SSW NNE

Geological Society of America Bulletin, Month/Month 2007 5 IN PRESS Solari et al.

A B

SSE NNW NNW SSE

C D

NNW SSE E F

NNW SSE NNW SSE G H

NNW SSE NNW SSE

6 Geological Society of America Bulletin, Month/Month 2007 IN PRESS Cretaceous–Tertiary magmatic and structural evolution, Tierra Colorada area, southern Mexico

Figure 4. (A) Polished hand sample of the Las Piñas granite. Section is parallel to L2 stretching lineation and perpendicular to the S2 mylonitic foliation. S-C texture is visible and indicates top-to-the north-northwest sense of shearing (black arrows). Scale is in centimeters.

(B) Polished hand sample of the Chapolapa metavolcanics, in its ultramylonitic facies. Section is parallel to L2 stretching lineation and per-

pendicular to the S2 mylonitic foliation. Top-to-the north-northwest sense of shearing is indicated by black arrows. Scale is in centimeters. (C) Photomicrograph of deformed quartz phenocrystals in the Chapolapa Formation. Quartz shows undulose extinction and formation of subgrains. Crossed-polarized light. (D) Rotated sanidine porphyroclast in the Chapolapa Formation. Counterclockwise rotation (in the

picture plane) and deﬂ ected S2 mylonitic foliation (center-bottom, left of the clast) generate a sigma-like structure, indicating a top-to-the north-northwest sense of shearing (white arrows). Crossed-polarized light. (E) Type 2b V pull-apart kinematic indicator (according to Samanta et al., 2002). Quartz ﬁ bers grow in the broken feldspar porphyroclast during counterclockwise opening, and indicate the maximum elongation in the shear plane. Shear sense is top-to-the north-northwest. Crossed-polarized light. (F) Photomicrograph of a sigma

object represented by a lithic fragment in the Chapolapa Formation. Tail asymmetry with respect to the S2 foliation (slightly left plunging in the picture plane, represented by the dark objects at the bottom) indicates a top-to-the north-northwest sense of shearing. Plane-polarized light. (G) Photomicrograph of a delta porphyroclast in the Chapolapa Formation. Counterclockwise rotation corresponds to a top-to-the north-northwest sense of shearing (white arrows). Crossed-polarized light. (H) Photomicrograph of a sigmoid biotite porphyroclast in the Las Piñas granite. Dextral displacement in the picture corresponds to a top-to-the north-northwest sense of shearing (white arrows). Plane- polarized light. Scale bars are 500µm.

which represent the last intrusive event recog- 1982), washed in warm 4M HNO3 for ~20 min, Applied corrections are 0.12% ± 0.04% for Pb nizable in the area. followed by 10 min in an ultrasonic bath. Fur- ratios, and 0.12% ± 0.05% for U ratios. Repeat ther hand-picking under binocular microscope analyses of 91500 zircon standard allow calcu- GEOCHRONOLOGY was undertaken to avoid crystals with surﬁ cial lation of U/Pb errors at ±0.5%. The Pb ratios contamination by pyrite remnants that could determined on concentrated feldspar separated Sample Preparation add common Pb to the analyses. Zircons were from the same samples were used to correct for then weighed on a microbalance, with an error initial common Pb in zircons. Reduction of raw μ In order to constrain the ages of structures and of ±1 g, washed again in 8M HNO3 on a hot data was performed using Pbdat (Ludwig, 1991), igneous events in the Tierra Colorada area, two plate for ~20 min, and put inside previously whereas concordia plots were performed using granitic samples for U-Pb geochronology, one ultraclean Teﬂ on microcapsules, together with Isoplot v.3.06 (Ludwig, 2004). Muscovite, K-

El Pozuelo granite and one Las Piñas granite, concentrated HF + HNO3 acids. As many as 9 feldspar, and whole-rock powder belonging to El were analyzed: 10–15 kg of rock sampled for microcapsules were stored in steel digestion ves- Salitre, as well as biotite and whole-rock pow- dating were crushed and powdered to <500 μm. sels, and samples were digested in an oven for 4 der of Las Piñas, were also processed for Rb-Sr Heavy minerals were then concentrated using a days at 240 °C. Ultrapure acids are used through geochronology. Biotite from Las Piñas was also Wilfl ey table, and further magnetic and nonmag- the digestion and chemical separation, and are dated by K-Ar. The Rb-Sr samples were also netic minerals were separated using a Frantz iso- normally obtained by Tefl on distilling follow- analyzed at LUGIS, UNAM. Rb isotope ratios dynamic magnetic separator. Zircons were con- ing the method described by Mattinson (1972). were measured with an NBS type single col- centrated and separated from other nonmagnetic Digested samples are poured into ultraclean PFA lector mass spectrometer (Teledyne Model SS- minerals such as quartz, feldspars, and apatite Tefl on beakers, spiked with a 205Pb/235U solution, 1290), whereas Sr isotopic measurements were using methylene iodide heavy liquid. Zircons evaporated until dry, and put again into a solu- performed in static mode on the Finnigan MAT were selected from the diamagnetic fraction at tion of 0.5M HBr acid. Chemical separation of 262. Samples were loaded as chlorides on double 2.0 amp by hand-picking under binocular micro- U and Pb was performed in 40 μL Tefl on micro- rhenium fi laments and measured as metallic ions. scope in ethanol. To assist interpretation, zircons columns fi lled with EIChrom AG1 X8 100–200 K-Ar samples were analyzed at LUGIS, UNAM, were observed and imaged under cathodolu- mesh anionic resin. The HBr-HCl chemical pro- as outlined in Ortega-Gutiérrez et al. (2004). minescence, using an ELM 3R luminoscope cedure is a modifi cation of the original method connected to a digital camera. One sample of (Krogh, 1973), and is similar to that described by Results El Salitre granite was also analyzed by Rb-Sr, Miller et al. (2006). Collected U and Pb aliquots and deformed micas of the protomylonitic por- are dried with a drop of 0.1M H3PO4, separately El Pozuelo Granite tion of Las Piñas granite were dated by Rb-Sr loaded on previously degassed Re fi laments, Zircons separated from the El Pozuelo gra- and K-Ar. Micas were separated starting from and measured with a Finnigan MAT 262 mass nitic sample are small, generally euhedral, col- 0.4 and 0.75 amp magnetic fractions, although spectrometer at the Laboratorio Universitario de orless or slightly yellow-amber, and prismatic fi nal selection was performed by hand-picking Geología Isotópica (LUGIS) of the Universidad to stubby, with a maximum elongation ratio under binocular microscope. Mica analyses fol- Nacional Autónoma de Mexico (UNAM). Pb is of 4:1. Crystals >200 μm are generally full of low analytical procedures described in the fol- measured in static mode, using Faraday cups for inclusions, and thus not suitable for dating, lowing and in Schaaf et al. (2000) for Rb-Sr, and 205Pb, 206Pb, 207Pb, and 208Pb beams, and an SEM so grains <200 μm were selected for analysis. in Ortega-Gutiérrez et al. (2004) for K-Ar. (secondary electron multiplier) for the smaller Cathodoluminescence (CL) controlled images 204Pb beam. Faraday-SEM counting gain was sta- show a general predominance of igneous zon- Analytical Procedures ble throughout the turret, with an error of 0.01%. ing in all the zircon morphologies; the zoning Routine analyses of NBS 981, 983, and U500 is sometimes developed around darker, oscilla- Selected zircons were normally abraded for 6– standards were performed to check precision tory zoned xenocrystic cores (Fig. 5). We dated 8 h using pyrite crystals as the abrasive (Krogh, and to correct instrumental mass fractionation. one large population of 50 prismatic grains, as

Geological Society of America Bulletin, Month/Month 2007 7 IN PRESS Solari et al.

XO0201-B 200µ m XO0201-L

XO0201-IXO0201-H XO0201-P XO0201-M 200µ m

200µ m 150µ m Xo0201 200µ m Lv0321

120 µm

LV0321-6 LV0321-7

200 µm 200µ m

LV0321-8 LV0321-A

200µ m 200µ m

Figure 5. Zircon photomicrographs and cathodoluminescence images of dated samples. El Pozuelo granite zircons are those labeled as XO0201, whereas Las Piñas zircons are labeled LV0321. Black and white bars indicate the scale. See also Table 1 for zircon descriptions.

8 Geological Society of America Bulletin, Month/Month 2007 IN PRESS Cretaceous–Tertiary magmatic and structural evolution, Tierra Colorada area, southern Mexico

well as 4 small populations, composed of 3–12 Pb

206 crystals, and a single crystal. All but one (frac-

Pb/ tion L in Table 1 and Fig. 5) were abraded prior 207

to analysis. Whereas three of the dated fractions ††† U are slightly discordant, the other three are con- 235

Pb/ cordant within analytical error, and the pris- 207 matic, single grain XO0201-I yielded an age of

U 129 ± 0.5 Ma (Fig. 6A). We interpret this to be 238 the age of crystallization of El Pozuelo granite. Pb/ 206 a de Mexico, Mexico City. a de Mexico, Mexico

Age (Ma) Age Las Piñas Granite Zircons separated from Las Piñas granite mposition are from isotopic are from analyses mposition of Pb (LV0321) are generally elongate prisms, color- 206 less or slightly yellow-amber, with an elongation Pb/ 207

ratio as high as 6:1 (Fig. 5). Pyramidal termina-

†† tions are also generally present, with well-devel- U

235 oped euhedral facets (e.g., sample LV0321-8 in Pb ratios are <0.8%, generally better than 0.1%; uncertainties 0.1%; uncertainties than generally better <0.8%, are Pb ratios 18957 0.0511691 0.0511691 18957 65 21 70 ± 248 Pb/

206 Fig. 5). CL images obtained from selected crys- 207 Pb/ tals mounted from this sample show a continu- 208 ous oscillatory zoning indicative of a magmatic U

238 origin, sometimes around an inherited core (see Pb and Pb/ CL images in Fig. 5). We dated four populations ype microcapsules. microcapsules. ype 206 206 y grains; brk xls—broken crystals.grains;to the micrometric y Numbers refer size of the

of zircons, one of which was abraded (fraction Pb/ 207 Atomic ratios Atomic 8 in Table 1 and Fig. 5). These populations are composed of prismatic crystals, more or less

Pb elongated, sometimes with pronounced pyra- 208

3896 0.0492532 0.0659436 0.0097104 62 36 65 ± 160 midal terminations (LV0321-A, LV0321-6, and Pb/ 206

† LV0321-7 in Fig. 6 and Table 1), and composed of colorless to pale yellow grains. In at least one IN TIERRA COLORADA AREA, SOUTHERN MEXICO AREA, SOUTHERN COLORADA IN TIERRA Pb processing Pb blank were between 10 and 40 pg. Initial Pb co Pb Initial pg. 40 and 10 between were blank Pb processing 207

case (LV0321-7) a clear inherited component 0800 8.7237 0.0211899 0.179326 0.061378 135 167 652 ± 6 0800 0.061378 0.179326 0.0211899 135 8.7237 ± 167 652 7291 5.3567 0.011464 0.0854742 0.0540753 73 83 374 ± 24 ± 73 0.0540753 83 374 0.0854742 0.011464 7291 5.3567 .

. 206 238 3 Pb/

9 represented by older ages ( Pb/ U age of 1 206 135.2 Ma) may indicate the presence of a xeno-

Observed ratios d Mattinson (1987) in Parrish (1987) t (1987) in Parrish (1987) d Mattinson 0 6 crystic core, possibly belonging to Xolapa-like 2 1 Pb . . 3 7 204

0 6 zircons (cf. zircon data in Ducea et al., 2004). A 8 2 Pb/ further fraction, made up of abraded pyramidal 206 U = 137.88. Uncertainities on the U/Pb ratio are 0.5%. 0.5%. are U/Pb ratio the on Uncertainities U = 137.88.

235 tips broken off larger, euhedral crystals (LV0321- U/

8 in Fig. 5 and Table 1) better constrains the 238 0 0

0 5 Pb spiked fraction. Two sigma uncertainties on the on uncertainties Two sigma Pb spiked fraction. Pb 1 1 (pg) lower intercept (Fig. 6B). All the data are dis- 205

Common cordant; however, the four populations deﬁ ne a

§ chord with a lower intercept of 54.2 ± 5.8 Ma.

3 1 925.61 20.2820 7.9253 0.0202257 0.135985 0.0487625 129 43 129 ± 136

3 0 Using just the lowermost three points yields a 1 1 28.6 18 540.90 18.2950 7.3042 0.0195386 0.0504037 0.135787 125 31 129 ± 214 (ppm) lower intercept of 57.6 ± 1.7 Ma (Fig. 6B and Total Pb inset). Conservatively, we assume the age of 54.2

3 2 . . § U = 9.48485 x 10E – 10; – 10E x U = 9.48485 9 9 ± 5.8 Ma as representing the age of intrusion of

6 0 U 235 5 7 chemistry are modified after Krogh (1973) an (1973) Krogh after modified chemistry are

(ppm) the Las Piñas granite. TABLE 1. U-Pb ANALYSES FOR SELECTED SAMPLES SAMPLES FOR SELECTED ANALYSES U-Pb TABLE 1. Whereas the volcanic rocks of the Chapo- §

t 4 3 h 2 8 lapa Formation underwent intense ﬂ uid circula- g

i 1 0 . . e (mg) 0 0 tion and were affected by intense hydrothermal easured on a Finnigan MAT 262 mass spectrometer with Secondary Electron Multiplier Ion Counting at Universidad Nacional Autónom Nacional Universidad at Counting Ion Multiplier Electron with Secondary mass spectrometer 262 MAT a Finnigan easured on W and from the data reduction program PbDat of Ludwig (1991). Total Total (1991). of Ludwig PbDat program reduction data the from and

σ recrystallization that transformed all the biotite into chlorite, biotite of the Las Piñas granite, although deformed (e.g., Fig. 4H), is unaltered. U = 1.55125 x 10E – 10; 10; – x 10E U = 1.55125

238 K-Ar analyses of such biotite (Table 2) yielded an age of 50.5 ± 1.2 Ma, whereas Rb-Sr (bio- W W ″ ″ tite–whole-rock isochron) yielded an age of 04 33 ′ ′ 45.3 ± 1.9 Ma (Fig. 6D).

Pb ratio vary from 0.1% to 2.4%. from 0.1% Pb ratio vary r r b b 204 Pb age uncertainties are 2 are uncertainties Pb age a a

206 El Salitre Granite Pb/

N, 99°33 N, 99°31 ″ ″

: Zircon sample dissolution and ion exchange ion exchange and dissolution sample : Zircon 7 6 ‡ 206 - - Pb/ Zircon is not present in the El Salitre granite; 1 1 43 12 ′ ′ 2 2 Decay constants used: All diamagnetic fractions at 2.0 amp. Xls—crystals; rnd—round; sh prsm—short prismatic to stubby grains; ov-stby—ovoid to stubb to ov-stby—ovoid grains; to stubby prismatic prsm—short sh rnd—round; Xls—crystals; amp. 2.0 fractions at All diamagnetic uncertainty. the weight to due ± 30%, at known are Concentrations Observed isotopic ratios are corrected for mass fractionation of 0.12% for 0.12% of fractionation corrected for mass are ratios Observed isotopic 3 3 Note †††207 ‡ § † †† analysis of two fractions of white mica, whole- 0 0

fraction chosen for analysis. for chosen fraction

feldspar separates. Isotopic data were m were data Isotopic separates. feldspar

in the V V L LV0321-8 abr tips, 6 brk pyr 0.049 775.8 9 100 211.77 9.2814 4. XO0201-M 10 xls, sh stby, clear, abr abr 0.026 clear, 819.7 stby, 20.9 sh 100 xls, 10 XO0201-M 236.35 9.7395 3.5790 0.0199844 0.0502834 0.138554 128 20 132 ± 208 El Pozuelo granite, sample XO0201 sample XO0201 granite, El Pozuelo XO0201-H abr sh prsm, 3 xls, clear, XO0201-I sng, prsm, abr 0.026 544.4 16.3 0.034 140 747.2 15. 125.18 6.8639 2.6603 0.0199644 0.135503 0.0492257 127 129 65 158 ± 17°05 XO0201-P 4 xls, long prsm, abr abr prsm, long xls, XO0201-P 4 0.033 541.2 11.6 16 641.22 16.9050 6.0369 0.0200642 0.135812 0.0490925 128 129 22 ± 152 17°06 Fraction XO0201-B abr prsm, clear, 50 xls, sh 0.100 1010.1 22.5 51 771.86 16.3180 6.1163 0.0207769 0.141940 0.0495475 133 135 16 174 ± sample LV0321 Las Piñas granite, L LV0321-A byp, yellow 24 xls, 0.124 901.9 10 150 449.53 12.5080 7.4485 0.0101905 0.07 XO0201-L prsm, clear 12 xls, 0.035 1419.8 rock powder, and K-feldspar (sample LV0136,

Geological Society of America Bulletin, Month/Month 2007 9 IN PRESS Solari et al.

150 150 A Las Piñas granite Lv0321 B 0.023 0.022 Intercepts at 54.2 ± 5.8 & 915 ± 67 Ma LV0321-8 130 140 MSWD = 6.0

0.021 0.018 XO0201-B 110 130 0.0120 XO0201-I XO 0201-P 76 U XO0201-H XO 0201-M U 238 XO0201-L 238 90 72 0.019 120 0.014 U Pb/ Pb/ 238 68 206 Pb/ 206 LV0321-7

70 206 110 0.017 El Pozuelo granite 0.010 LV0321 A 0.0100 64 LV0321-6 3-point intercepts at XO0201 57.6 ± 1.7 & 1146 ± 130 Ma 60 MSWD = 1.4 One concordant at 129 ± 0.5 Ma 50 100 0.06 0.08 207Pb/ 235U 0.015 0.006 0.10 0.11 0.12 0.13 0.14 0.15 0.16 0.04 0.08 0.12 0.16 0.20 207Pb/235U 207Pb/235U

0.718 C D 0.84 0.716

LV0136Mu1 LV0321 Bi 0.714 LV0136Mu2 0.80 0.712 Sr Sr 86 86 0.710 Sr/

0.76 Sr/ 87

87 0.708 - El Salitre granite, Lv0136 - - Las Piñas granite, Lv0321 - (Mu1-Mu2-Kfs-WR) 0.706 (Bi-WR)(Bi-WR) 0.72 Age = 55.3 ± 3.5 Ma LV0321 WR Age = 45.3 ± 1.9 Ma LV0136 WR Initial 87Sr/86Sr = 0.7087 ± 0.0012 Initial 87Sr/86Sr = 0.± 0.00008 LV0136 KF 0.704 MSWD = 27 MSWD = 0.000

0.68 0.702 0 40 80 120 160 200 0412 8 16 20 87 86 87Rb/86Sr Rb/ Sr Figure 6. U-Pb concordia diagrams and Rb-Sr isochron plots for the dated samples in the studied area. Ellipses for U-Pb concordia diagrams represent 2σ errors, whereas crosses in Rb-Sr isochrons are 1σ errors. Given errors in calculated ages are 2σ in both cases. MSWD—mean square of weighted deviates. Bi—biotite; WR—whole rock; Mu—muscovite; Kfs—K-feldspar.

TABLE 2. Rb-Sr AND K-Ar ANALYSES FOR SELECTED SAMPLES IN TIERRA COLORADA AREA, SOUTHERN MEXICO Concentration 40 –10* 40 (ppm) 87 86 87 86 K Ar* × 10 Ar* Age ± 2σ Sample Mineral/whole rock Rb/ Sr Sr/ Sr ± 1 SD 2 SE n Isotopic dilution (M) (%) (mol/g) (%) (Ma) Rb Sr El Salitre granite, sample LV0136 17°08′52.7″ N, 99°38′00.0″ W LV0136 Mu1 Muscovite > 1 mm 556.1 10.9 149.6 0.827377 ± 45 15 35 LV0136 Mu2 Muscovite 0.5 mm 556.3 13.0 125.1 0.805748 ± 48 16 35 LV0136 KF Potassic feldspar 165.7 174.5 2.7 0.710537 ± 37 10 54 LV0136 WR Whole rock 133.8 119.5 3.2 0.711552 ± 39 11 55 55.3 ± 3.5 Ma†

Las Piñas granite, sample LV0321 17°05′43″ N, 99°31′33″ W LV0321 Bi Biotite 567.5 110.1 14.9 0.714466 ± 37 10 56 6.64 5.896 93.1 50.5 ± 1.2 Ma LV0321 WR Whole rock 109.0 598.4 0.5 0.705210 ± 38 10 56 45.3 ± 1.9 Ma† Note: Rb and Sr element concentrations and isotopic ratios were determined with the same run each consisting of 60 isotopic ratios. N—number of measured isotopic ratios. SD—standard deviation. SE—standard error. The values (1SD = ±1σ abs) refer to the error during measurement, in the last two digits. σ σ 86 88 2 SE(M) = 2 abs/ n. All Sr isotopic ratios were corrected for mass fractionation by normalizing to Sr/ Sr = 0.1194. Laboratory values for standard NBS 987 87 86 σ σ 87 86 (SrCO3): Sr/ Sr = 0.710237 ± 21 (±1 abs, n = 317). Relative uncertainty (1 ) of Rb/ Sr = ±2 % (experience-based value for the LUGIS laboratory). Relative reproducibilities (1σ) for Rb and Sr concentrations at the laboratory are ± 4.5% and ± 1.8% respectively. Total procedure blanks during analyses of these samples were: 0.06 ng Rb and 1.5 ng Sr. *40Ar* is the radiogenic argon, expressed as moles/g and as percentage of radiogenic Ar from total Ar, respectively. †Numbers in italic are Rb-Sr mineral-WR cooling ages.

10 Geological Society of America Bulletin, Month/Month 2007 IN PRESS Cretaceous–Tertiary magmatic and structural evolution, Tierra Colorada area, southern Mexico

Fig. 6C and Table 2) yielded a Rb-Sr isochron El Pozuelo granite), the stretched quartz linea- D4: Open Folding with an age of 55.3 ± 3.5 Ma. Although a Rb- tion is parallel to a mineral lineation made up of Sr mineral–whole-rock isochron generally aligned biotite and hornblende crystals, indicat- A kilometer-scale, west-northwest–trending, indicates mineral cooling age, the large size of ing that while quartz was plastically stretched, gently eastward-plunging, upright synclinal F4 muscovite analyzed, between 0.5 and >2 mm, both biotite and hornblende only underwent fold affecting both the Chapolapa and Morelos together with the lack of deformation, suggests passive rotation toward the direction of maxi- Formations is present across the study area. It is that the calculated age closely postdates the time mum elongation. truncated by the Tierra Colorada pluton (Fig. 2, of intrusion. Microstructures in the volcanic rocks of the map and section). Chapolapa Formation, also described in detail STRUCTURAL EVOLUTION by Riller et al. (1992), show intracrystalline INTERPRETATION quartz deformation represented by deforma- Four phases of deformation have been recog- tion lamellae and subgrains, as well as undulose The combination of mapping and structural nized in the area. extinction (e.g., Fig. 4C). Zoned porphyroclasts data with the geochronology presented here present in the Chapolapa Formation have cores allows the following evolutionary history for the

D1: Migmatization and Deformation of the made up of Ca-rich plagioclase, and rims with Tierra Colorada area to be constructed.

Xolapa Complex more Na-rich plagioclase intergrown with seri- 1. The D1 deformation and low-pressure cite. A top-to-the north-northwest sense of shear metamorphism in the Xolapa Complex is syn- The oldest deformational fabric was only is indicated by: (1) the asymmetry in the zoned chronous with migmatization, which predates observed in the Xolapa gneisses, where it is porphyroclasts of the Chapolapa Formation, as intrusion of the 129 ± 0.5 Ma El Pozuelo gran- a penetrative high-grade foliation deﬁ ned by well as sigma and delta objects (Figs. 4F, 4G), ite. This is consistent with U-Pb zircon ages quartz + plagioclase ± K-feldspar microlithons combined with V pull-apart shear indicators (laser ablation–inductively coupled plasma mass alternating with biotite + hornblende foliated (type 2b of Samanta et al., 2002) within plagio- spectrometer analyses) of ca. 130–140 Ma (Cre- domains. Neosome lenses of quartz + plagioclase crystals (Fig. 4E); (2) S-C fabrics in the taceous) that Ducea et al. (2004) interpreted as clase ± feldspar ± biotite generally develop Las Piñas protomylonitic granite (e.g., Passchier being syntectonic with the main deformation parallel to S1, although they are not internally and Trouw, 1996); and (3) deformed biotite ﬁ sh event in the Xolapa basement. This event can foliated. In the western part of the area, between in both the Las Piñas granite and Chapolapa also be extrapolated to previous observations of

Xolapa and El Papagayo towns (Fig. 2), S1 Formation (Fig. 3H). Deformed quartz ribbons Herrmann et al. (1994) that dated one migmatite foliation is northwest trending and moderately are present in both the Chapolapa Formation belonging to the Xolapa Complex northwest of southwest dipping, whereas in the east (Villa and Las Piñas granite, and their microtextures Puerto Angel, Oaxaca, as 131.8 ± 2.2 Ma, disre- Guerrero to Omitlán, and west of El Zapote, suggest bulging recrystallization processes (e.g., garded as being speculative (cf. Herrmann et al., Fig. 2) it is northeast trending, and gently to Passchier and Trouw, 1996; Stipp et al., 2002). 1994, p. 462, 467). moderately southeast dipping. Neither mineral 2. Intrusion of the El Pozuelo granite and nor stretching lineation is present in the east- D3: South-Southwest–Verging Thrusting extrusion of the volcanic rocks of the Chapolapa ern area, and it is not pervasively affected by and Folding Formation occurred in the Cretaceous. Emplace- younger D2 or D3 events. ment of El Pozuelo granite at 129 ± 0.0.5 Ma A large, south-southwest–verging thrust (concordant U-Pb zircon age) provides a mini-

D2: Mylonitization and North-Northwest– places limestones of the Morelos Formation on mum age for the migmatization of the Xolapa Vergent Listric Normal Faulting top of volcanic rocks of the Chapolapa Forma- Complex. This result is at odds with Herrmann tion. The main fault plane is north-northeast et al. (1994), who inferred that migmatization in

The S2 foliation is the dominant ductile fabric trending, with a 40°–60°NE dip (Fig. 3D). the entire Xolapa Complex took place between in the area, mainly affecting volcanic rocks of The contact is made up of fault gouge as thick 46 and 66 Ma. The age obtained on El Pozuelo the Chapolapa Formation and metagranitoids as 5 m composed of both volcanic rocks and granite is similar to the 126–133 Ma U-Pb zir-

such as El Pozuelo and Las Piñas granites. In limestones. The S2 foliation in the Chapolapa con ages for the volcanic rocks of the Chapolapa the volcanic rocks of the Chapolapa Formation, Formation is deformed by west-trending, sub- volcanic rocks reported by Campa and Iriondo foliation is west to west-southwest trending with horizontally plunging, decimeter- to meter- (2004) and Hernández-Treviño et al. (2004),

gently northward dips north of the dam (just scale F3 folds, whose s-shaped asymmetry suggesting that they belong to the same associa- north of La Venta Vieja in Fig. 2), and close to the indicates a top-to-the south sense of shear tion of subduction-related magmas. contact with the overlying Morelos limestones, (Fig. 3G). Other top-to-the southwest kine- 3. Deposition of the Morelos Formation whereas steeper dips are generally found south matic indicators are small thrusts developed in occurred during the Albian–Cenomanian.

of the dam. In the El Pozuelo granite, S2 is south- chert lenses (Fig. 3E) and stair-stepping indi- 4. Late Paleocene intrusion of both Las Piñas

west trending and moderately to steeply dipping cators (Fig. 3F). F3 kink bands also affect the and El Salitre granites was ca. 55 Ma (U-Pb

toward the northwest (Fig. 2, stereonet). S2 mylonitic foliation in the Chapolapa Forma- zircon lower intercept age, and Rb-Sr isochron, In the Las Piñas protomylonitic granite, the tion, just a few meters below the thrust con- respectively). few studied outcrops show a west- to west- tact, and their kinematics are compatible with 5. Petrographic observations carried on the

northwest–trending, north to north-northeast D3 thrusting (Fig. 3C). The Morelos Formation microtextures described in the D2 mylonites of

moderately dipping S2 foliation. An accompa- lacks evidence of ductile shearing at its base, the Chapolapa Formation and Las Piñas granite nying, north-northwest– to northwest-trending, being only affected by brittle deformation. This suggest that deformation took place at ~300 °C.

moderately plunging L2 stretching lineation is is interpreted as further evidence that it was not Given that deformed biotite crystals are ~150– generally made up of stretched quartz ribbons. in contact with the underlying Chapolapa For- 300 μm, and assuming closure temperatures for

Locally, in the less deformed granitoids (e.g., mation during D2 (cf. Riller et al., 1992). Ar and Sr isotopic systems of ~270–290 °C, the

Geological Society of America Bulletin, Month/Month 2007 11 IN PRESS Solari et al.

300 °C deformation should have opened the 9. There is a post-Eocene, brittle, left-lateral ously reported as ca. 158–165 Ma (Guerrero- biotite isotopic system for both Ar and Sr sys- strike-slip to extensional regime in the Tierra García et al., 1978; Ducea et al., 2004) and 28– tematics (Harrison et al., 1985; Shirley, 1991). Colorada pluton aureole (fault striae from Riller 34 Ma (e.g., Herrmann et al., 1994; Schaaf et The calculated ages of 45–50 Ma (Rb-Sr and et al., 1992). al., 1995; Ducea et al., 2004; Hernández-Pineda, K-Ar mineral ages), in spite of the slight age 2006), they constitute an episodic sequence of difference that could be explained by either DISCUSSION AND SUMMARY subduction-related magmatic pulses roughly excess Ar or probably disturbed systematics forming every 25–30 m.y. The hiatus ca. 90– in the Rb-Sr whole rock used to construct the The age data and structural reconstruction 100 Ma in the Tierra Colorada area and, more isochron, constrain mylonitization of the La of the Tierra Colorada area allow integration in general, between Zihuatanejo to the west and

Venta area. This event produced S2 mylonitic of some of the previously reported data for the Huatulco to the east (Fig. 1), corresponds to the foliation and L2 mineral stretching lineation, evolution of the Sierra Madre del Sur and pro- deposition of the Morelos Formation (Albian– with kinematic indicators giving a top-to-the vide a more coherent interpretation of the tec- Cenomanian), as well as oceanic backarc mag- north-northwest sense of shearing along a north- tonics of southwestern Mexico (Fig. 7). matism at the Arcelia-Palmar Chico basin in the northeast– dipping fault. Our age limitations for Guerrero terrane, just northwest of the studied mylonitization refi ne those inferred by Riller et Age of Migmatization of the Xolapa Complex area (93–103 Ma; Sánchez-Zavala, 1993; Elías- al. (1992) of 90–34 Ma. Herrera et al., 2000; Mendoza and Suastegui, 6. The late Paleocene–early Oligocene Balsas In the Acapulco–Tierra Colorada transect, 2000). Such features can be related to a series of Formation was deposited on top of the alloch- an Early Cretaceous upper limit on the age geodynamic processes, the fi rst of which would thonous Morelos Formation. Because the Balsas of migmatization in the Xolapa Complex is be the arrival of a continental block that choked Formation is only present on top of the More- strongly delimited by the crystallization age the paleotrench, roughly during the Early Creta- los Formation in the studied area, not covering of ca. 129 Ma obtained in the unmigmatized ceous. Such accretion and/or collision could have directly either the thrust or the basement, and it El Pozuelo granite, dikes of which cut across led to an interruption of subduction and hiatus in is not cut by the D3 thrust, the time of its local migmatitic gneisses. magmatism between ca. 130 and 60 Ma. Colli- sedimentation with respect to the regional defor- sion would also be coherent, with clockwise mation is still poorly defi ned. Timing of Arc-Related Magmatism, pressure-temperature-time paths necessary,

7. Eocene D3 southwest-directed thrusting Magmatic Hiatus Circa 100 Ma, and according to Corona-Chávez et al. (2006), to and D4 east-west–trending folding occurred Platformal Sedimentation develop high-grade fabrics and migmatization between 45 and 34 Ma. in the Xolapa Complex. This continental block 8. Late Eocene–early Oligocene intrusion We recognize two magmatic episodes in the could be the Guerrero terrane, which, according of the Tierra Colorada granite occurred at 34 Tierra Colorada area, at 129 ± 0.5 and ca. 55 Ma. to Campa and Coney (1983) and Dickinson and ± 2 Ma (Herrmann et al., 1994). Together with other magmatic episodes previ- Lawton (2001), was accreted to nuclear Mexico

Age (Ma) Magmatism and sedimentation Tectonic and/or deformation event Inferred subducted slab geometry

MI OC 23.8 Decrease in slab dip

OL IGOCENE Tierra Colorada Increase in slab dip 33.7 Upright folding magmatism D4

CENOZOIC SSW thrusting D3 Balsas Fm. EOCENE D La Venta 2 Decrease in slab dip 54.8 El Salitre - Las Piñas shear zone Increase in slab dip magmatism NNW extension from ca. 68 to ca. 54 Ma

PALEOC 65.0 Laramide Orogeny Flat slab subduction LATE 99.0 Morelos Fm. No subduction: passive margin development Chortís extension and rifting

EARLY El Pozuelo magmatism

CRETACEOUS Chortís accretion to S Mexico Choking of the trench 144

LATE Foliation/ D1 159 Mid Jurassic magmatism ? MES OZOI C banding/ migmatization

MI DDL E 180 Xolapa Complex JURASSIC

EARLY 206 Figure 7. Summary diagram showing the main magmatic and structural events recognized in the studied area, using the new data presented in this work, as well as those referenced in the text (in italics). Inferences about the geometry of the subducted slab are also shown.

12 Geological Society of America Bulletin, Month/Month 2007 IN PRESS Cretaceous–Tertiary magmatic and structural evolution, Tierra Colorada area, southern Mexico during the Early Cretaceous; however, such a Figueroa et al. (2003) and Levresse et al. (2004). Schmidt (1983), Pindell et al. (1988), Ross and hypothesis was criticized by Elías-Herrera and Collectively, these events indicate a new onset Scotese (1988), Herrmann et al. (1994), Schaaf Ortega-Gutiérrez (1998), because of the absence of subduction and thermal perturbation in the et al. (1995), Meschede et al. (1996), and Nieto- of an exposed suture east of the Guerrero ter- continental margin, and a time of magmatism Samaniego et al. (2006) suggests that Chortís rane. Alternatively, the continental block could migration from the continent interior toward the displacement from southern Mexico occurred have been the Chortís block, which, according to coast. This could be indicative of a change in dip during the Eocene–Miocene, based on the pres- paleomagnetic data of Gose (1985), was located of the downgoing slab, which would increase ence of east-west left-lateral to transtensional in front of southwestern Mexico ca. 140 Ma. A from ca. 62 to 54 Ma. faults, as well as the migration of magmatism. similar position for the Chortís block during the Farther southeast, the cooling ages of Oligo- However, other data indicate that Chortís trans- Early Cretaceous was proposed by Harlow et al. cene–Miocene plutons dated by Schulze et al. lation was characterized by a transpressional (2004), who studied coeval eclogites now found (2004) between Puerto Angel and Huatulco sug- regime (e.g., Cerca et al., 2004; Morán-Zenteno in the southern Motagua mélange, Guatemala. gest a northward younging, i.e., in the opposite et al., 2005), and that its collision with the south- According to Harlow et al. (2004), the exhu- direction with respect to the Acapulco–Tierra ern Maya block occurred between 65 and 73 Ma mation of such eclogites would have occurred Colorada segment. Thus, a decrease in the dip of (e.g., Harlow et al., 2004; Ortega-Gutiérrez et al., at 125–115 Ma, following collision of Chortís the subduction zone during the Oligocene–Mio- 2004). Moreover, Keppie and Morán-Zenteno with southern Mexico, i.e., well before the onset cene southeast of Tierra Colorada might be indi- (2005) argued that convergence of Chortís with of the Laramide orogeny. We propose that soon cated. As pointed out by Morán-Zenteno et al. the Maya block occurred by a translation from after its collision Chortís became an extensional (2005), the oblique convergence of the Farallon west-southwest to east-northeast, and that Chor- arc that produced backarc rifting with a passive plate, its retreat during the Oligocene–Miocene tís was located far southeast of southern Mexico margin on its northeastern side ca. 100–110 Ma. (e.g., Nieto-Samaniego et al., 1999), and its shal- during the Eocene. Such development would have been in agree- lowing (e.g., Morán-Zenteno et al., 1999) are all If Chortís separated from southern Mexico ment with the backarc magmatism described factors that can contribute to the migration of before the Tertiary, those features developed dur- here, and predated platformal sedimentation of magmatic ages, and the formation and propaga- ing Paleocene–Miocene time, e.g., the 46–50 Ma the Morelos Formation. tions of east-west, strike-slip left-lateral faults. detachment along the Tierra Colorada shear zone, as well as the southwest-directed thrusting Time of Laramide deformation in Southern Tertiary Brittle Deformation of the Morelos Formation and other left-lateral Mexico to extensional structures, must be interpreted in

Our data indicate that D3, south-southwest– a different way. In our view, they may be related The platformal sedimentation was followed directed thrusting of the Morelos Formation to any combination of stress transmission dur- by the Laramide orogeny, roughly bracketed on top of the Chapolapa Formation postdates ing mechanical coupling between subducting between 93 and 87 Ma (Hernández-Romano Laramide deformation and Paleocene sub- and overlying plates associated with changes in et al., 1997; Lang and Frerichs, 1998), and duction-related magmatism in southwestern dip of the subduction zone, as those evidenced 55–60 Ma, the age of postorogenic, subduction- Mexico. Such shortening is widespread in the herein (e.g., Fig. 7), combined with the irregular related magmatism (González-Partida et al., eastern Morelos Guerrero platform, where it geometry of the subducted slab and the possible 2003; Levresse et al., 2004; Cerca et al., 2004; forms a wide range of folds and generally west- subduction of aseismic ridges as proposed by this paper). By analogy with western North to southwest-verging thrusts (e.g., Cerca et al., Keppie and Morán-Zenteno (2005). The same America, the origin of the Laramide orogeny 2004, 2007). This contrasts with Riller et al. processes can be responsible for the supposed in southern Mexico, expressed by contractile (1992) and Meschede et al. (1996), who did not end of slightly diachronic magmatism along structures of the Morelos Formation, can be a recognize such thrusting event, but grouped the the margin of southern Mexico (cf. Ducea et combination of several factors, such as fl atten- 40–70 Ma events as left-lateral movements on al., 2004), previously interpreted as evidence of ing of the subducting slab (English et al., 2003, the Chacalapa–La Venta fault zone, in terms of the Tertiary migration of the Chortís block (e.g., and references therein), or increasing conver- an east-west subhorizontal σ1 and a north-south Herrmann et al., 1994; Schaaf et al., 1995). gence rate (e.g., Saleeby, 2003; English and subhorizontal σ3. It also contrasts with Nieto- Plutonism in the Xolapa terrane and in the Johnston, 2004). Samaniego et al. (2006), who did not consider the Tierra Colorada area ended by ca. 30 Ma and ca. 34–47 Ma shortening and southwest-verging subduction erosion of the forearc led to exhu- Post-Laramide Magmatism event, and generalized that all the contractional mation of the present continental margin (e.g., structures were Laramide. Our structural and Morán-Zenteno et al., 1996; Ducea et al., 2005; Paleocene granitoids that do not show any geochronology data indicate that normal-left Keppie et al., 2007). evidence of contractional deformation are wide- lateral displacement along the La Venta shear spread in southern Mexico. In the Tierra Colo- zone predates such compressional event; i.e., ACKNOWLEDGMENTS rada area these are represented by Las Piñas and they are kinematically different events. El Salitre granites (ca. 55 Ma; this study), simi- Fruitful discussions with A. Gómez-Tuena, M. lar to the ca. 59 Ma in pegmatites reported by Post-Laramide Tectonics Elías-Herrera, F. Ortega-Gutiérrez, M. Cerca, and J.D. Keppie allowed clarifi cation of several impor- Morán-Zenteno (1992). In Acapulco, ~50 km tant aspects of southern Mexico geology. This paper southwest of the studied area, an undeformed Post-Laramide widespread magmatism and benefi ted from funds granted to Solari (CONACyT syenitic granite was dated as ca. 54 Ma by zir- deformation are present in Tierra Colorada and, # J-39783) and PAPIIT-DGAPA, UNAM (Programa con U-Pb (Ducea et al., 2004). About 80 km generally, in Southern Mexico. We speculate de apoyo para la investigación e innovación tec- nológica, Dirección General de Asuntos del Personal north of the studied area, 63–66 Ma (zircon U- that these features are the expression of major Académico, Universidad Nacional Autónoma de Pb and Ar-Ar), granitic to granodioritic postde- plate reorganization after the removal of the México; IN101407). We thank P. Schaaf, J.J. Morales, formation magmatism was described by Meza- Chortís block. Previous work by Anderson and and M.S. Hernández-Bernal from Laboratorio

Geological Society of America Bulletin, Month/Month 2007 13 IN PRESS Solari et al.

Universitario de Geología Isotópica for assistance formed near a spreading center: Comment: Tecto- Keppie, J.D., 2004, Terranes of Mexico revisited: A 1.3 and for their participation during analytical work nophysics, v. 292, p. 321–326, doi: 10.1016/S0040- billion year odyssey: International Geology Review, and data acquisition. Careful reviews by R. Molina, 1951(98)00051-1. v. 46, p. 765–794. L. Ratschbacher, and an anonymous reviewer greatly Elías-Herrera, M., and Ortega-Gutiérrez, F., 2002, The Calte- Keppie, J.D., and Morán-Zenteno, D.J., 2005, Tectonic pec Fault Zone: An Early Permian dextral transpres- implications of alternative Cenozoic reconstructions improved the manuscript. sional boundary between the Proterozoic Oaxacan and for southern Mexico and the Chortís Block: Interna- Palaeozoic Acatlán complexes, southern Mexico, and tional Geology Review, v. 47, p. 478–491. 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