Sedimentary Xenoliths of the Mesozoic Nicoya Complex, Costa Rica – Data Record from the Playa Arbolito Forearc Basement Assemblage, Nicoya Peninsula
Description
The dataset consists of ten individual research data files over the Cretaceous sedimentary xenolith field of Playa Arbolito, exposed on the western Nicoya Peninsula of Costa Rica (Fig. S1). The data record a unique assemblage of radiolarite, radiolarian chert, chert, siliceous tuffaceous mudstone, and glassy pyroclastic xenoliths hosted in basalt of the Mesozoic forearc basement succession (Fig. S2-S3), called the Nicoya Complex after Dengo (1962). The data were gathered during two different field research campaigns in February 2012 and March 2017. While the 2012 research centered on xenoliths and their host rocks found at Playa Arbolito (Fig. S1), the 2017 campaign dealt in addition with potential xenolith source-rock strata contained in Cretaceous sedimentary sections of the interior Peninsula and NE Gulf of Nicoya coast (e.g., Sardinal, Manzanillo sections) (Table S1). Further key data come from detailed micropaleontological, sedimentological, and volcanological rock examinations in outcrop and laboratory. The dataset includes furthermore a newly detailed examination of Schmidt-Effing’s (1979) and Gursky’s (1984) xenolith rock collections from this locality. They are described and illustrated with full color images in dataset DS1. Despite marine erosion of rock exposures, xenolith samplings in different years still show similar lithologies at Playa Arbolito. As Figure S8 records, boring activity of pholadid bivalves in the intertidal zone actually contributes to active bioerosion of basement exposures, including pyroclastic xenoliths. The micropaleontological data show the notable first record of a Turonian planktic foraminiferal assemblage in marine pyroclastic rock from Nicoya Complex led by the index species Helvetoglobotruncana helvetica (Bolli) (Fig. S5). This find dates Cretaceous highly differentiated explosive arc volcanism in Costa Rica, similarly to that recorded by pyroclastic layers of the nearby Albian to Campanian Loma Chumico Formation, a sedimentary forearc basin succession (Calvo and Bolz, 1994; Calvo, 1998). The unique basalt-hosted xenolith assemblage including sedimentary tuffaceous lithologies, particularly highly differentiated biotite- and hornblende-bearing pyroclastic rocks, evidence bimodal volcanic activity linked to Cretaceous island arc volcanism on the western Caribbean Plate boundary. The variety of sedimentary xenolith lithologies also found in Mesozoic strata sections of Nicoya Peninsula and NE Gulf of Nicoya points out to xenolith formation caused by widespread and long-term (~140 to 70 Ma) intrusion of basalt throughout the Cretaceous deep-sea forearc basin succession. They also show that 90 Ma aged Tortugal komatiite lavas erupted coevally with explosive arc volcanism. These data contribute to understand the nature of igneous forearc basement rock exposed along the Pacific coast of southern Central America, and also provide a useful reference for future research in active forearc and arc settings.
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A total of 31 hand specimens of sedimentary and igneous rocks from basement exposures at Playa Arbolito and the interior Nicoya Peninsula were selected for detailed petrographical, microfacies, and micropaleontological analysis. They were collected during sedimentological and volcanological field investigations. Nearly all samplings deal with sedimentary rock xenoliths and their basaltic host rocks. Sample examination was particularly concerned with pyroclastic rocks and their age determination based on contained planktic foraminifera. Thin sections of all rock samples were prepared for further examination under the microscope both in plane- and cross-polarized light in order to determine composition, texture, microfacies, and microfossil contents. The newly examined samples of Schmidt-Effing’s and Gursky’s collections (dataset DS1) consist of 17 thin sections, from which 15 are actually permanently stored in the authors' collection in Stuttgart, Germany. They include two consecutive sample series numbered 276 and 277. The twelve samples of our 2012/2017 collections are labelled JPA-1 (IGSN:IECCV0001) to JPA-12 (IGSN:IECCV000E) (Table S1). The abbreviation JPA stands for the initials of “Junquillal Playa Arbolito”, the general name of sampling site. Dating of pyroclastic xenolith samples was established by using micropaleontological methods based on direct microscope examination of well-preserved foraminiferal specimens in random and oriented thin sections of hard rock samples JPA-2 (IGSN:IECCV0002) and JPA-3 (IGSN:IECCV0003). Detailed images of index species are presented in Figs. S4-S7. The micropaleontological dating focused on marine finer-grained pyroclastic rocks where microfossil contents are usually higher. This method was crucial in order to avoid masked radiometric dating of pyroclastic materials (e.g., biotite and hornblende) that result from metamorphic overprint. Planktic foraminiferal identification follows original taxonomic descriptions and illustrations for Mesozoic species from the online database PF@Mikrotax at https://www.mikrotax.org/pforams/index. All examined and dated samples got IGSN’s registered within the System for Earth Sample Registration (SESAR) (Table S1, DS1). In biostratigraphical terms, the age determination provides an accurate Turonian age of explosive volcanic arc activity within the Helvetoglobotruncana helvetica zone (92.99 to 93.48 Ma, Anthonissen and Ogg, 2012; Falzoni et al., 2018). This is also the maximum age of reference for the intrusive basaltic activity that produced the xenolith assemblage in this part of the peninsula. In particular biotite- and hornblende-bearing samples were selected for geochemical analysis. Sedimentary xenolith describes in this dataset a fragment of sedimentary or volcaniclastic rock enclosed in basaltic rock. Sedimentary xenoliths appear altered, mostly recrystallized due to contact metamorphism with host igneous rock, often exhibiting black glassy coverings (Fig. S2).