Andrés Ulloa - Academia.edu (original) (raw)

Conference Presentations by Andrés Ulloa

En el río Corredores (579.711/289.086, hoja topográfica Canoas, escala 1:50 000 ) aflora una secu... more En el río Corredores (579.711/289.086, hoja topográfica Canoas, escala 1:50 000 ) aflora una secuencia de areniscas finas a medias, bien estratificadas en estratos decimétricos, color gris verdoso, que contiene una gran cantidad de moluscos, crustáceos y echinodermos de muy buena preservación. Esta secuencia se correlaciona con la Formación Charco Azul, Mioceno-Plioceno.
Los moluscos son el grupo con mayor biodiversidad, presentando alrededor de 17 especies de bivalvos y 24 de gasterópodos.
Los bivalvos en su mayoría se encuentran con las valvas conjugadas y algunas especies (Nuculana, Tivela, Ostrea, Noetia) en posición vital y alineadas posiblemente a favor de las corrientes. Hay que destacar un nivel con Ostrea (Lopha) haitiensis, valvas alineadas, que constituye un buen indicador paleobatimetrico.
Los gastrópodos son principalmente especies carnívoras, que se alimentaron de la abundante fauna de bivalvos existente (algunos presentan icnofósiles tipo Praedicnia: Oichnus sp.).
Los crustáceos están representados por caparazones de cangrejos, que constituyen mudas bastante completas y tenazas aisladas. Los echinodermos se encuentran como placas de Echinoidea aisladas y espículas.
El depósito puede interpretarse como una biofacies vital lipostrate, constituída por una biocenosis de fondo, con organismos que vivieron en la zona sublitoral profundo y por tafocenosis de organismos de aguas más profundas (Typhis).

Papers by Andrés Ulloa

Two caves located on the NW sector of the main crater of Irazú volcano are described for the fi r... more Two caves located on the NW sector of the main crater of Irazú volcano are described for the fi rst time. The caves, called Mucolitos and Minerales, are located at the base of the landslide t on December 8 th , 1994, and are found in pyroclastic rocks with intense hydrothermal alteration. We suggest that both caves are associated with spaces between blocks in block slide with stepped surface and this mechanism is proposed as new vulcanospeleological mechanism. We collected twenty samples which were analyzed based on x-ray diffractometry studies and their physical properties. Twenty one differents minerals corresponding to twenty sulfates and one native element were identifi ed. Five of these minerals are reported for the fi rst time in worldwide caves (aplowite CoSO 4 •4H 2 O, boyleite (Zn, Mg) SO 4 ·4H 2 O, ferrinatrite Na 3 Fe(SO 4 ) 3 •3H 2 O, magnesiocopiapite MgFe +3 4 (SO 4 ) 6 (OH) 2 •20H 2 O and wupatkiitea (Co,Mg, Ni)Al 2 (SO 4 ) 4 •22H 2 O). The cave presents different speleothemes, the crust is the most common one, but also stalactites, stalagmites and geysermites are present. In both caves, the presence of snottites formations (extremophile bacteria colonies) was identifi ed. We propose four principals mineralogenic mechanisms for the formation of these minerals, which correspond to degassing, solubilization alteration and dehydratation. The infl uence of biogenic activity in the precipitation of some minerals is also suggested.

En la falda suroeste de la cordillera de Talamanca aflora un batolito del Mioceno Superior con co... more En la falda suroeste de la cordillera de Talamanca aflora un batolito del Mioceno Superior con composiciones de gabro-dioriticas a graníticas pertenecientes al Grupo Intrusivo Talamanca, el cual es uno de los pocos cuerpos intrusivos del mundo desarrollado en un arco de islas. En general los granitos y granodioritas corresponden a series calco alcalinas hasta alcalinas, mientras que los gabros y las dioritas son más cálcicos. Los intrusivos son metalumínicos a ligeramente peralumínicos y correlacionados a un ambiente tectónico de arco de islas, presentan valores de SiO2 y Mg# entre los rangos de una corteza continental (55-75% y 40-60 respectivamente). El patrón de elementos traza de los granitos y granodioritas es similar al de la corteza continental superior. El Grupo Intrusivo Talamanca se caracteriza por presentar plagioclasas con valores de An entre 12-52% (andesinas y oligoclasas), feldespatos alcalinos clasificados principalmente como sanidina, piroxenos cálcicos, hornblendas...

Although tourism is presently the main source of income of the Republic of Costa Rica, making an ... more Although tourism is presently the main source of income of the Republic of Costa Rica, making an analysis of the offer and demand of the topic of "underground sites as tourist attractions", it is evident that in our country this activity is minimal, with percentage figures that are not even taken into account in the statistics. At government level, there's only one National Park whose focus is caves (Barra Honda); in this aspect, there's also very little and ambiguous legislation. At a private enterprise level, there are only five karstic underground sites worthy of mention, of which only two can qualify as 'business operations'. The other 3 are underground sites to which occasionally and informal visits are launched, but it is still difficult to find references, even in the web.

Revista Geológica de América Central, 2014

Nowadays the technological advances are revolutionizing the way and efficiency of work and its im... more Nowadays the technological advances are revolutionizing the way and efficiency of work and its impact is being reflected in the geological sciences. The new microelectromechanical systems allow the incorporation of a wide variety of sensors in devices such as smartphones and tablets. These instruments are multifunctional and highly portable for data collection and replace others devices in the field as personal computers and PDA. Currently, high precision GPS in conjunction with LIDAR technology and sonar technology are more accessible for geological research. With this technology is possible to generate high-resolution three-dimensional models to complement geological studies. A remote sensing technique like high penetration radar is used for modeling ice thickness and bedrock topography in Antarctic. Modern tridimensional scanning and printing techniques are used in geological research and teaching. Also the advance in computer technology allows handling three-dimensional models on personal computers very efficiently and with different display options.

Revista Geológica de América Central, 2011

In Costa Rica the karstic phenomenon affects mainly carbonate rocks, constituting 0.85% of the co... more In Costa Rica the karstic phenomenon affects mainly carbonate rocks, constituting 0.85% of the country's surface, with an approximate area of 431 km 2 . There are documented at least 9 episodes of carbonate deposition from the Late Cretaceous to Holocene, which include eleven stratigraphic units. These events are related to various geological-tectonic processes, changes in sea level, subsidence and changes in the sediment input. Ten karstic zones and four limestone sites have been defined: 1) North Pacific, 2) Tempisque, 3) South Zone, 4) Central Pacific, 5) Cabo Blanco and Malpaís, 6) Turrialba and Pacuare, 7) Talamanca, 8) Venado , 9) Central Valley, 10) Limón and the isolated karst areas: A) Garza, B) Cerro Huevo Frito, C)Cerro Morro and D) Playa Panama. These karstic features have morphologies which can be exo-karstic, including lapiaz, 'mogotes', karstic-towers, sinkholes and conical karst. The best known endokarstic phenomena are caves, in which it is common the development of speleothems. such as stalactites, stalagmites, columns, flowstones, draperies, calcareous pearls, etc. Serpiente Dormida (Sleeping Snake) cave is the deepest in the country, with -172 m, the longest is the Bruja -Rectangle -Tururún -Corredores system, with 3872 m in length. At least 258 caves in limestone have been registered, of which 41% have been mapped (100 caves). 88% of the caves in Costa Rica are found in karst areas of the Southern Zone (61%), Tempisque (17%) and Venado (10%), while the remaining 12% of caves are distributed in other karstic areas. These areas have special environmental and hydrogeological conditions that must be taken into account as to infrastructure development, and could also be exploited by tourism.

Revista Geológica de América Central, 2009

En una sección del río Corredores aflora una secuencia de areniscas finas a gruesas en estratos (... more En una sección del río Corredores aflora una secuencia de areniscas finas a gruesas en estratos (dm-m) subverticales orientados predominantemente N-S. Se reconocieron 43 especies de moluscos, crustáceos, fragmentos de equinodermos e icnofósiles de muy buena preservación. Esta secuencia se correlaciona con la Formación Curré. Con base en los foraminíferos planctónicos se data la base de la secuencia como Mioceno Temprano (zonas N4-N5), por lo que se propone ampliar el rango de la Formación a todo el Mioceno. Los moluscos son el grupo con mayor biodiversidad, presentando alrededor de 25 especies de gasterópodos y 18 de bivalvos. Se describe una especie nueva: Nuculana (Saccella) corredorensis. Los bivalvos en su mayoría se encuentran en posición vital (Nuculana, Ostrea) y en niveles bien definidos, principalmente de Ostras. Los gasterópodos son principalmente especies carnívoras y se alimentaron de la abundante fauna de bivalvos produciendo ichnofosiles tipo Praedicnia: Oichnus isp.. Los crustáceos están representados por carapaces de cangrejos, que constituyen mudas bastante completas y tenazas aisladas. Los Equinoidea incluyen placas aisladas y espículas. Los icnofósiles están representados por Thalassinoides isp. y Oichnus isp. El depósito representa una biofacies vital lipostrate, constituida por una biocenosis de fondo, con organismos que vivieron en la zona sublitoral y acumulación de organismos de otras comunidades conformando eventos tempestíticos.

Revista Geológica de América Central, 2011

During the night of December 8 th , 1994, along an unstable area at the northern fl ank of Irazú ... more During the night of December 8 th , 1994, along an unstable area at the northern fl ank of Irazú volcano, Costa Rica, a small volcanic debris avalanche and subsequent debris fl ows took place, followed by an "ashfall" that reached 30 km southwest from the volcano. At that time, these processes were interpreted as the result of a phreatic explosion. However, a review of the descriptions given in the technical reports and other documents of the time, together with the analysis of the geomorphology obtained from aerial photographs taken on December 20, 1994, by the Costa Rica National Geographic Institute, lead to discuss the validity of this interpretation, which although not ruled out, it seems unlikely. This paper proposes, based on the facts and information available, a new hypothesis of the occurrence of a volcanic debris avalanche resulting from a major landslide near the northern summit area. Consequently, the following "ash" fall could be interpreted as generated by the dust produced byauto-fragmentation of the debris avalanche, so subsequently called co-debris avalanche dust-cloud deposit.

Journal of GEOsciences, 2012

The Guacimal Pluton is situated in the Cordillera de Tilarán in the northwestern Costa Rica. It f... more The Guacimal Pluton is situated in the Cordillera de Tilarán in the northwestern Costa Rica. It forms an oval-shaped body strongly elongated in the NW-SE direction. Its dimensions are ~15 × 4-6 km with an exposed surface of 60-70 km 2 . The pluton intruded basic volcanic rocks of the Aguacate Group (Miocene-Pliocene) and is surrounded by a wide thermal aureole of calc-silicate metasomatic rocks. The pluton is mainly formed of monzogranites to granodiorites, which strongly prevail over more basic types occuring scarce and relatively thin dykes and enclaves. The dominant magmatic minerals of this felsic suite are quartz, plagioclase, and K-feldspar with subordinate Mg-rich biotite, amphibole I, and magnetite. Orthopyroxene, Mn-rich ilmenite, Al-poor titanite, rutile, apatite, zircon, thorite, and chalcopyrite are accessories. Secondary minerals, which occur as fillings of miarolitic cavities and interstices, are quartz II, K-feldspar II, epidote, chlorite, actinolite, ilmenite II and Al-rich titanite II. The much less frequent mafic suite (mainly quartz diorite to quartz monzodiorite/monzogabbro) is composed of plagioclase, pargasite, actinolite, K-feldspar, quartz and magnetite, with accessory amounts of opaque minerals, epidote, chlorite, and titanite. The pluton was emplaced at a depth of c. 3 km, crystallized at temperature of c. 760-800 °C under a relatively high oxygen fugacity (1.6-2.1 log units above the NNO buffer). Increased activities of volatiles (H 2 O, F) upon cooling are indicated by the presence of highly aluminous, F-rich titanite and other hydrous silicates in miarolitic cavities. The prevailing, felsic rocks of the Guacimal Pluton are high-K calc-alkaline, whereas the mafic suite is nearly exclusively medium-K calc-alkaline in nature. Laser ablation ICP-MS dating of zircons from two granite samples yielded statistically identical U-Pb ages of 6.3 ± 0.5 and 6.0 ± 0.4 Ma, respectively. The Sr-Nd isotopic compositions are rather primitive ( 87 Sr/ 86 Sr 6 = 0.70380-0.70413, ε 6 Nd ~ +7.3 to +7.9). Narrow range of these values rules out open-system processes such as magma mixing or assimilation of isotopically contrasting upper continental crust. Instead, the felsic suite is interpreted as either having crystallized from a highly fractionated melt extracted from a plagioclase-amphibole-dominated crystal mush in a putative deep crustal reservoir or a product of partial melting of older arc-related rocks, such as intermediate lavas or volcaniclastics or immature psammitic sediments rich in volcanic material. The observed variation in the felsic suite was most likely produced by low degrees of closed-system fractional crystallization of an assemblage dominated by feldspars. At least some of the rocks of the volumetrically subordinate mafic suite may represent lithologies rich in the complementary cumulates. The online version of this article (doi: 10.3190/jgeosci.087) contains supplementary electronic material.

En el río Corredores (579.711/289.086, hoja topográfica Canoas, escala 1:50 000 ) aflora una secu... more En el río Corredores (579.711/289.086, hoja topográfica Canoas, escala 1:50 000 ) aflora una secuencia de areniscas finas a medias, bien estratificadas en estratos decimétricos, color gris verdoso, que contiene una gran cantidad de moluscos, crustáceos y echinodermos de muy buena preservación. Esta secuencia se correlaciona con la Formación Charco Azul, Mioceno-Plioceno.
Los moluscos son el grupo con mayor biodiversidad, presentando alrededor de 17 especies de bivalvos y 24 de gasterópodos.
Los bivalvos en su mayoría se encuentran con las valvas conjugadas y algunas especies (Nuculana, Tivela, Ostrea, Noetia) en posición vital y alineadas posiblemente a favor de las corrientes. Hay que destacar un nivel con Ostrea (Lopha) haitiensis, valvas alineadas, que constituye un buen indicador paleobatimetrico.
Los gastrópodos son principalmente especies carnívoras, que se alimentaron de la abundante fauna de bivalvos existente (algunos presentan icnofósiles tipo Praedicnia: Oichnus sp.).
Los crustáceos están representados por caparazones de cangrejos, que constituyen mudas bastante completas y tenazas aisladas. Los echinodermos se encuentran como placas de Echinoidea aisladas y espículas.
El depósito puede interpretarse como una biofacies vital lipostrate, constituída por una biocenosis de fondo, con organismos que vivieron en la zona sublitoral profundo y por tafocenosis de organismos de aguas más profundas (Typhis).

Two caves located on the NW sector of the main crater of Irazú volcano are described for the fi r... more Two caves located on the NW sector of the main crater of Irazú volcano are described for the fi rst time. The caves, called Mucolitos and Minerales, are located at the base of the landslide t on December 8 th , 1994, and are found in pyroclastic rocks with intense hydrothermal alteration. We suggest that both caves are associated with spaces between blocks in block slide with stepped surface and this mechanism is proposed as new vulcanospeleological mechanism. We collected twenty samples which were analyzed based on x-ray diffractometry studies and their physical properties. Twenty one differents minerals corresponding to twenty sulfates and one native element were identifi ed. Five of these minerals are reported for the fi rst time in worldwide caves (aplowite CoSO 4 •4H 2 O, boyleite (Zn, Mg) SO 4 ·4H 2 O, ferrinatrite Na 3 Fe(SO 4 ) 3 •3H 2 O, magnesiocopiapite MgFe +3 4 (SO 4 ) 6 (OH) 2 •20H 2 O and wupatkiitea (Co,Mg, Ni)Al 2 (SO 4 ) 4 •22H 2 O). The cave presents different speleothemes, the crust is the most common one, but also stalactites, stalagmites and geysermites are present. In both caves, the presence of snottites formations (extremophile bacteria colonies) was identifi ed. We propose four principals mineralogenic mechanisms for the formation of these minerals, which correspond to degassing, solubilization alteration and dehydratation. The infl uence of biogenic activity in the precipitation of some minerals is also suggested.

En la falda suroeste de la cordillera de Talamanca aflora un batolito del Mioceno Superior con co... more En la falda suroeste de la cordillera de Talamanca aflora un batolito del Mioceno Superior con composiciones de gabro-dioriticas a graníticas pertenecientes al Grupo Intrusivo Talamanca, el cual es uno de los pocos cuerpos intrusivos del mundo desarrollado en un arco de islas. En general los granitos y granodioritas corresponden a series calco alcalinas hasta alcalinas, mientras que los gabros y las dioritas son más cálcicos. Los intrusivos son metalumínicos a ligeramente peralumínicos y correlacionados a un ambiente tectónico de arco de islas, presentan valores de SiO2 y Mg# entre los rangos de una corteza continental (55-75% y 40-60 respectivamente). El patrón de elementos traza de los granitos y granodioritas es similar al de la corteza continental superior. El Grupo Intrusivo Talamanca se caracteriza por presentar plagioclasas con valores de An entre 12-52% (andesinas y oligoclasas), feldespatos alcalinos clasificados principalmente como sanidina, piroxenos cálcicos, hornblendas...

Although tourism is presently the main source of income of the Republic of Costa Rica, making an ... more Although tourism is presently the main source of income of the Republic of Costa Rica, making an analysis of the offer and demand of the topic of "underground sites as tourist attractions", it is evident that in our country this activity is minimal, with percentage figures that are not even taken into account in the statistics. At government level, there's only one National Park whose focus is caves (Barra Honda); in this aspect, there's also very little and ambiguous legislation. At a private enterprise level, there are only five karstic underground sites worthy of mention, of which only two can qualify as 'business operations'. The other 3 are underground sites to which occasionally and informal visits are launched, but it is still difficult to find references, even in the web.

Revista Geológica de América Central, 2014

Nowadays the technological advances are revolutionizing the way and efficiency of work and its im... more Nowadays the technological advances are revolutionizing the way and efficiency of work and its impact is being reflected in the geological sciences. The new microelectromechanical systems allow the incorporation of a wide variety of sensors in devices such as smartphones and tablets. These instruments are multifunctional and highly portable for data collection and replace others devices in the field as personal computers and PDA. Currently, high precision GPS in conjunction with LIDAR technology and sonar technology are more accessible for geological research. With this technology is possible to generate high-resolution three-dimensional models to complement geological studies. A remote sensing technique like high penetration radar is used for modeling ice thickness and bedrock topography in Antarctic. Modern tridimensional scanning and printing techniques are used in geological research and teaching. Also the advance in computer technology allows handling three-dimensional models on personal computers very efficiently and with different display options.

Revista Geológica de América Central, 2011

In Costa Rica the karstic phenomenon affects mainly carbonate rocks, constituting 0.85% of the co... more In Costa Rica the karstic phenomenon affects mainly carbonate rocks, constituting 0.85% of the country's surface, with an approximate area of 431 km 2 . There are documented at least 9 episodes of carbonate deposition from the Late Cretaceous to Holocene, which include eleven stratigraphic units. These events are related to various geological-tectonic processes, changes in sea level, subsidence and changes in the sediment input. Ten karstic zones and four limestone sites have been defined: 1) North Pacific, 2) Tempisque, 3) South Zone, 4) Central Pacific, 5) Cabo Blanco and Malpaís, 6) Turrialba and Pacuare, 7) Talamanca, 8) Venado , 9) Central Valley, 10) Limón and the isolated karst areas: A) Garza, B) Cerro Huevo Frito, C)Cerro Morro and D) Playa Panama. These karstic features have morphologies which can be exo-karstic, including lapiaz, 'mogotes', karstic-towers, sinkholes and conical karst. The best known endokarstic phenomena are caves, in which it is common the development of speleothems. such as stalactites, stalagmites, columns, flowstones, draperies, calcareous pearls, etc. Serpiente Dormida (Sleeping Snake) cave is the deepest in the country, with -172 m, the longest is the Bruja -Rectangle -Tururún -Corredores system, with 3872 m in length. At least 258 caves in limestone have been registered, of which 41% have been mapped (100 caves). 88% of the caves in Costa Rica are found in karst areas of the Southern Zone (61%), Tempisque (17%) and Venado (10%), while the remaining 12% of caves are distributed in other karstic areas. These areas have special environmental and hydrogeological conditions that must be taken into account as to infrastructure development, and could also be exploited by tourism.

Revista Geológica de América Central, 2009

En una sección del río Corredores aflora una secuencia de areniscas finas a gruesas en estratos (... more En una sección del río Corredores aflora una secuencia de areniscas finas a gruesas en estratos (dm-m) subverticales orientados predominantemente N-S. Se reconocieron 43 especies de moluscos, crustáceos, fragmentos de equinodermos e icnofósiles de muy buena preservación. Esta secuencia se correlaciona con la Formación Curré. Con base en los foraminíferos planctónicos se data la base de la secuencia como Mioceno Temprano (zonas N4-N5), por lo que se propone ampliar el rango de la Formación a todo el Mioceno. Los moluscos son el grupo con mayor biodiversidad, presentando alrededor de 25 especies de gasterópodos y 18 de bivalvos. Se describe una especie nueva: Nuculana (Saccella) corredorensis. Los bivalvos en su mayoría se encuentran en posición vital (Nuculana, Ostrea) y en niveles bien definidos, principalmente de Ostras. Los gasterópodos son principalmente especies carnívoras y se alimentaron de la abundante fauna de bivalvos produciendo ichnofosiles tipo Praedicnia: Oichnus isp.. Los crustáceos están representados por carapaces de cangrejos, que constituyen mudas bastante completas y tenazas aisladas. Los Equinoidea incluyen placas aisladas y espículas. Los icnofósiles están representados por Thalassinoides isp. y Oichnus isp. El depósito representa una biofacies vital lipostrate, constituida por una biocenosis de fondo, con organismos que vivieron en la zona sublitoral y acumulación de organismos de otras comunidades conformando eventos tempestíticos.

Revista Geológica de América Central, 2011

During the night of December 8 th , 1994, along an unstable area at the northern fl ank of Irazú ... more During the night of December 8 th , 1994, along an unstable area at the northern fl ank of Irazú volcano, Costa Rica, a small volcanic debris avalanche and subsequent debris fl ows took place, followed by an "ashfall" that reached 30 km southwest from the volcano. At that time, these processes were interpreted as the result of a phreatic explosion. However, a review of the descriptions given in the technical reports and other documents of the time, together with the analysis of the geomorphology obtained from aerial photographs taken on December 20, 1994, by the Costa Rica National Geographic Institute, lead to discuss the validity of this interpretation, which although not ruled out, it seems unlikely. This paper proposes, based on the facts and information available, a new hypothesis of the occurrence of a volcanic debris avalanche resulting from a major landslide near the northern summit area. Consequently, the following "ash" fall could be interpreted as generated by the dust produced byauto-fragmentation of the debris avalanche, so subsequently called co-debris avalanche dust-cloud deposit.

Journal of GEOsciences, 2012

The Guacimal Pluton is situated in the Cordillera de Tilarán in the northwestern Costa Rica. It f... more The Guacimal Pluton is situated in the Cordillera de Tilarán in the northwestern Costa Rica. It forms an oval-shaped body strongly elongated in the NW-SE direction. Its dimensions are ~15 × 4-6 km with an exposed surface of 60-70 km 2 . The pluton intruded basic volcanic rocks of the Aguacate Group (Miocene-Pliocene) and is surrounded by a wide thermal aureole of calc-silicate metasomatic rocks. The pluton is mainly formed of monzogranites to granodiorites, which strongly prevail over more basic types occuring scarce and relatively thin dykes and enclaves. The dominant magmatic minerals of this felsic suite are quartz, plagioclase, and K-feldspar with subordinate Mg-rich biotite, amphibole I, and magnetite. Orthopyroxene, Mn-rich ilmenite, Al-poor titanite, rutile, apatite, zircon, thorite, and chalcopyrite are accessories. Secondary minerals, which occur as fillings of miarolitic cavities and interstices, are quartz II, K-feldspar II, epidote, chlorite, actinolite, ilmenite II and Al-rich titanite II. The much less frequent mafic suite (mainly quartz diorite to quartz monzodiorite/monzogabbro) is composed of plagioclase, pargasite, actinolite, K-feldspar, quartz and magnetite, with accessory amounts of opaque minerals, epidote, chlorite, and titanite. The pluton was emplaced at a depth of c. 3 km, crystallized at temperature of c. 760-800 °C under a relatively high oxygen fugacity (1.6-2.1 log units above the NNO buffer). Increased activities of volatiles (H 2 O, F) upon cooling are indicated by the presence of highly aluminous, F-rich titanite and other hydrous silicates in miarolitic cavities. The prevailing, felsic rocks of the Guacimal Pluton are high-K calc-alkaline, whereas the mafic suite is nearly exclusively medium-K calc-alkaline in nature. Laser ablation ICP-MS dating of zircons from two granite samples yielded statistically identical U-Pb ages of 6.3 ± 0.5 and 6.0 ± 0.4 Ma, respectively. The Sr-Nd isotopic compositions are rather primitive ( 87 Sr/ 86 Sr 6 = 0.70380-0.70413, ε 6 Nd ~ +7.3 to +7.9). Narrow range of these values rules out open-system processes such as magma mixing or assimilation of isotopically contrasting upper continental crust. Instead, the felsic suite is interpreted as either having crystallized from a highly fractionated melt extracted from a plagioclase-amphibole-dominated crystal mush in a putative deep crustal reservoir or a product of partial melting of older arc-related rocks, such as intermediate lavas or volcaniclastics or immature psammitic sediments rich in volcanic material. The observed variation in the felsic suite was most likely produced by low degrees of closed-system fractional crystallization of an assemblage dominated by feldspars. At least some of the rocks of the volumetrically subordinate mafic suite may represent lithologies rich in the complementary cumulates. The online version of this article (doi: 10.3190/jgeosci.087) contains supplementary electronic material.