Manuscript accepted for publication in Geol. Soc. London Special Paper "HP-UHP Metamorphism and Tectonic Evolution of Orogenic Belts" Petrography, mineralogy and geochemistry of jadeite-rich artefacts from the Playa Grande excavation site, northern Hispaniola: Evaluation of local provenance from the Río San Juan Complex Hans-Peter Schertl1,2*, Walter V. Maresch1, Sebastiaan Knippenberg3, Andreas Hertwig1,4, Adolfo López Belando5, Reniel Rodríguez Ramos6, Laura Speich1,7, Corinne L. Hofman3 1
Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, 44780
Bochum, Germany 2
College of Earth Science and Engineering, Shandong University of Science and Technology,
Qingdao, Shandong 266590, China 3
Faculteit Archeologie, Caribbean and Amazonia, Reuvensplaats 3-4, 2311 BE Leiden, The
Netherlands 4
now at Department of Geoscience, University of Wisconsin-Madison, Madison, WI, USA
5
Calle Fantino Falco No. 47, Edificio Naco 2000, Suite 804, Santo Domingo, Rep.
Dominicana 6
Universidad de Puerto Rico, Recinto de Utuado, Utuado, Puerto Rico
7
now at School of Earth Sciences, University of Bristol, Queen's Road, Bristol BS8 1RJ, UK
* Correspondence (
[email protected])
Running title: “Jadeitite artefacts from Playa Grande, DR”
Abstract Many archeological sites with jadeitite artefacts are known in the Caribbean region, but defining the source of the raw material is a major problem, because of great mineralogical heterogeneity both in potential sources and in artefacts. The archaeological settlement site of Playa Grande on the northern coast of the Dominican Republic is particularly significant, because it yielded evidence of on-site axe manufacture, and lies only 20-30 km NE of a recently discovered potential source area of serpentinite mélanges in the near-by Río San Juan Complex (RSJC). A suite of nine artefacts was chosen from a collection of over 100 excavated woodworking tools rich in jadeite, as well as two blueschist artefacts. Permission to perform destructive analysis allowed data on petrography, mineral chemistry and bulk-rock chemistry to be obtained. Seven of the nine artefacts are jadeitite s.str. (> 90 vol.% jadeite),
which are identical to material known from the RSJC. Two artefacts are jadeite-lawsonite rocks. These and the two blueschists show only minor differences from corresponding rocks of the RSJC source. With this direct linking of source and site material it is now possible to better define source discriminators for the Caribbean and to assess sampling bias.
During the fifties and sixties of the last century, laboratory experiments combined with petrological study of samples in nature clearly showed that the stability field of the pyroxene jadeite (NaAl[Si 2 O 6 ]), and of pyroxenes rich in this component (as in the omphacite common in eclogite) called for high-pressure/low-temperature (HP/LT) conditions of metamorphism not considered possible at that time. Studies on jadeite thus helped to usher in a new era of plate-tectonic interpretation of global processes, associated with an increased understanding of high-pressure (HP) and ultimately ultra-high-pressure (UHP) metamorphism in subduction and collision zones. On the other hand, jadeite in monomineralic aggregates forms jadeitite, the rarest variety of jade, which has been a material of special significance within different regions worldwide since prehistoric times (e.g., Keverne 2010; Harlow et al. 2014). The combination of being very tough and hard, in addition to its brilliance when polished, have made it a very desirable raw material not only for manufacturing tools, but also for creating highly valued objects of adornment and worship (see different contributions in Keverne 2010). Jadeitite jade is the subject of this paper and is a rock that is technically defined as consisting of more than 90 vol. % of the pyroxene jadeite, usually also containing minor amounts of diopside (CaMg[Si 2 O 6 ]), hedenbergite (CaFe2+[Si 2 O 6 ]) and aegirine (NaFe3+[Si 2 O 6 ]) components in solid solution, but in practice it is often found that the term jadeitite is used loosely for any jadeite-rich rock. The second, very much more common variety of jade is nephrite, a rock which essentially consists of an amphibole of the tremoliteactinolite-ferroactinolite solid-solution series (Ca 2 (Mg,Fe) 5 [(OH) 2 ǀSi 8 O 22 ]). Nephrite is a rock type with a very different context of formation (Harlow et al., 2014) and lacks the clear link to plate-tectonic processes of jadeitite jade (Harlow et al. 2015). About 20 jadeitite occurrences are now known worldwide (Harlow et al. 2014), all of which are genetically related to serpentinite mélanges in subduction-zone settings. The use of jadeitite has traditionally been affiliated with the complex civilizations of southeast Asia and Meso- and Central America, where long-term exploitation and utilization have given this scarce semi-precious stone an important role in local cosmology, ritual, and value systems
(see Lange 1993; Graham et al. 1998; Keverne 2010; Helferich 2012; Harlow et al. 2014). Recent research has emphasized that jadeitite had also been exploited, used and valued in other parts of the world, such as for example Neolithic Europe and the pre-Columbian Antilles (Harlow et al. 2006; Rodríguez Ramos 2007, 2010; Pétrequin et al. 2012; GarcíaCasco et al. 2013). Due to this high validation, which in part results from its scarcity, jadeitite has always been an object of (elite) exchange (Lange 1993; Graham et al. 1998). Given the small number of source localities and the often extensive distribution of jadeitite objects observed within particular regions of the world, archaeological provenance studies on jadeitite have provided unique information on prehistoric long-distance interaction and exchange systems (see for example Pétrequin et al. 2012). Because jadeitites are often nearly monomineralic in composition, it has previously been shown that only a very detailed petrographical, mineralogical and geochemical characterization may be productive in finding those characteristics which distinguish sources from each other and allow assigning artefacts to them. However, the difficulty of unambiguously assigning an archeological artefact to a specific field locality remains to this day, due to considerable mineralogical and geochemical intra-source variations found in potential jadeitite source localities (e.g., García-Casco et al., 2013). The Caribbean represents a region where the pre-colonial use of jadeitite has received more and more attention during the past decades. The available data suggest that it represented a much utilized and widely distributed raw material for the manufacture of woodworking tools such as axes, adzes and chisels (Fig. 1a). Only very recently have three new local Caribbean source regions been discovered, mapped, and petrologically studied. Garcia-Casco et al. (2009) and Cárdenas Párraga et al. (2012) describe jadeitite from the Sierra del Convento region of eastern Cuba. Maresch et al. (2012) present a detailed study of a possible occurrence in the Escambray area of central Cuba briefly mentioned by Millán & Somin in 1981. The third is located in the Río San Juan Complex of the northern Dominican Republic (Schertl et al. 2007a, 2007b, 2012; Hertwig, 2014; Hertwig et al. 2015, 2016). Prior to these discoveries, it had already been recognized that jadeitite was being used as a raw material and formed part of the extensive networks of human mobility and exchange maintained by the indigenous peoples inhabiting the Caribbean islands before the arrival of Columbus (Hofman et al. 2010, 2014). The authors of initial provenance studies on multiple jadeitite artefacts from a number of different Caribbean islands, including Puerto Rico and Antigua (Harlow et al. 2006; Rodríguez Ramos 2007, 2010), were not then aware of these
local Caribbean sources and concluded that the extensive and important Meso-American jadeitite source region of the Motagua fault zone in Guatemala (McBirney, 1967; Harlow, 1994; Seitz et al., 2001; Harlow et al., 2003, 2004, 2011) likely was the origin of these Caribbean artefacts, suggesting that jadeite was important in the circum-Caribbean networks of interaction and exchange from the Early Ceramic Age on (Rodríguez Ramos 2007, 2010; Hofman et al. 2010, 2011). These new local sources were incorporated during a later study in which the petrological, mineralogical, and geochemical data from a new series of artefacts excavated on the Lesser Antillean island of St. Eustatius were compared to the now available augmented source data (García-Casco et al. 2013). The outcome, however, still favored the Motagua fault zone over the Cuban and Dominican sources. A proper evaluation of whether the geographically closer Dominican Republic locality might have been the source was at that time hampered by still limited data available from this latter occurrence (Schertl et al. 2012). The data gap on the jadeitites found in the Río San Juan Complex has now been filled by the studies of Hertwig (2014) and Hertwig et al. (2016). However, in the meantime, excavation and analysis of numerous jadeitite artefacts (López Belando 2012; Knippenberg 2012, Knippenberg et al. 2012) from the Playa Grande site, a Late Ceramic Age settlement site along the northern coast in close proximity to the Río San Juan complex (Fig.1b), has added a new aspect to these provenance studies. The importance of the Playa Grande site with regard to jadeitite studies in the Caribbean is two-fold: (a) it represents the first reported archaeological site within the Caribbean very close to a jadeitite source (López Belando 2012; Knippenberg 2012; Knippenberg et al. 2012); (b) it is the first site within the Caribbean for which it has been shown that jadeitite had locally been manufactured into tools (Knippenberg 2012; Knippenberg et al. 2012). This latter fact, combined with the predominance of jadeitite artefacts among the wood-working tools at Playa Grande, clearly suggests that the local community exploited the near-by jadeitite occurrence. This suggestion has important implications for Caribbean jadeitite provenance studies. Instead of only seeing the Playa Grande site as a consumer of axes, we can assume that it also has been a producer, and may therefore have been an important source community from where axes to other sites had been exchanged (Breukel, 2017). The specific aim of the present paper therefore is to carefully characterize -petrographically, mineralogically and geochemically -- a sample suite of jadeitite artefacts from the Playa Grande site, using polarizing microscopy and cathodoluminescence imaging, in addition to mineral and bulk-rock analysis, in order to develop mineralogical and geochemical fingerprints. This will be followed by a close comparison with the now available
extensive data set of jadeitites from the Río San Juan Complex (Schertl et al. 2012; Hertwig 2014; Hertwig et al. 2015, 2016). The Río San Juan Complex jadeitites are rare and characterized by an extensive intra-source mineralogical variability (Schertl et al., 2012; Hertwig 2014; Hertwig et al. 2015, 2016), and, likewise, the jadeitite axes show a range of compositions. Thus it cannot be excluded that some specific jadeitite varieties may have been overlooked during the geological survey, or that perhaps varieties that have been exhausted are therefore now absent in present-day outcrops or secondary source localities. The outcome of the current study should clarify whether the artefacts did indeed originate from the near-by source locality of the Río San Juan Complex, or whether there may be indications that some or all of the material might have come from more distant sources such as Cuba or Guatemala (Harlow 1994; Harlow et al. 2003, 2004, 2011; Seitz et al., 2001; García-Casco et al., 2009, 2013; Cárdenas Párraga et al. 2012; Maresch et al., 2012). In a further more archeologically oriented paper, these results will be evaluated against current views on the distribution trajectories the artefacts may have followed and on what implications this might have for our understanding of the long-distance exchange networks in the Caribbean region (cf., García-Casco et al. 2013; Harlow et al. 2006; Rodríguez Ramos 2010, 2011; Hofman et al. 2010, 2011).
The Playa Grande excavation site
The archaeological settlement site of Playa Grande is located on the northern coast of the Dominican Republic (UTM Zone 19Q, 394109 2176057), about 8 km east of the town of Río San Juan (Fig. 1b). It comprises a multi-component site with an extensive period of occupation starting from about 800 cal AD until it was abandoned at about 1600/1700 cal AD (López Belando 2012). This long-term settlement is reflected in the presence of a wide variety of ceramic types, belonging to the Ostionoid, Meillacoid and Chicoid series and covering the entire Ceramic Age in the Dominican Republic (López Belando 2012). Evidently the Playa Grande inhabitants concentrated on the manufacture of woodworking tools. Study of the lithic assemblage showed that the excavations unearthed more than three hundred axes and related tools, such as adzes and chisels (Knippenberg 2012; Knippenberg et al. 2012). Part of the material consists of unfinished pre-forms, suggesting these woodworking tools were locally made. Careful analysis of these pre-forms showed that water-worn cobbles likely from river beds had been collected as raw material. The absence of any other manufacturing debitage in the form of flakes and shatter indicates that these cobbles had been shaped only by pecking,
followed by a final grinding and polishing stage. Traces of pecking are found on many of the pre-forms and some of the finished tools. The fact that many finished axes still partly possess unmodified natural water-worn surfaces supports this notion of a minimal manufacturing process. This becomes fully understandable when it is realized that the rock types that were chosen are very hard and tough, making it a difficult and time-consuming task to work and shape them. The most predominant rock-types among the woodworking tools are jadeite-rich, making up 36%, thus comprising more than one hundred specimens in the excavated assemblage. Most of the other rock types observed can also be classified as metamorphic. These, however, exhibit a broad range of rock varieties, also comprising metasediments and metavolcanics.
The jadeitite-bearing serpentinite-mélanges of the Río San Juan Complex
The Río San Juan Complex (RSJC) is an inlier of igneous and metamorphic rocks (Draper & Nagle 1991; Escuder-Viruete 2010; Escuder-Viruete et al. 2013) extending about 50 km towards the south from the town of Río San Juan (Fig. 1b) and with a maximum width of ca. 25 km. Serpentinite mélanges were originally mapped by Draper & Nagle (1991) in the northern part of this complex (Fig. 1b) and were shown to entrain a heterogeneous suite of high-pressure metamorphic blocks. The RSJC in general and the serpentinite mélanges in particular are related to an intra-oceanic subduction zone that swept relatively eastward through the widening gap between North and South America in the Cretaceous (see Pindell & Kennan 2009; Boschmann et al. 2014; and many references therein) and sutured both arc and HP/LT metamorphic material onto the northern and southern continental borderlands. This “Great Arc of the Caribbean” (Burke 1988) marked the prow of a lobe of Pacific oceanic crust overriding newly-formed Proto-Caribbean oceanic crust between North and South America. The Great Arc was probably initiated by 135 Ma (Pindell et al. 2012). Its present position is represented by the Lesser Antilles Island Arc. Based on the results of a major new 1:50,000 mapping project, Escuder-Viruete (2010) and Escuder-Viruete et al. (2013) concluded that the northern RSJC should be interpreted as a tectonic stack of imbricate slabs originally derived from a subduction-related metamorphic complex, which was later involved in a large-scale fold-and-thrust system offset by numerous high-angle faults. Krebs et al. (2008, 2011) showed that the serpentinite mélanges represent a fossil subduction-zone channel that was active before at least 120 Ma and functioning until 55
Ma, when collision with the North American continental margin shut down the subduction process. Krebs et al. (2008, 2011) documented innumerable blocks of various metamorphic rock types such as blueschist, eclogite, jadeitite and jadeite-rich rock, cymrite-bearing rock, marble, metapelite, and orthogneiss, and described their pressure-temperature-time paths in a thermally evolving and cooling subduction zone. The jadeite-bearing rocks that are the focus of this study are found as blocks up to about 2m x 2m x 2m in size in lag deposits on the hilly terrain of the RSJC, or as reworked boulders within mountain streams. Schertl et al. (2012) presented a first systematic classification of the jadeitites and jadeite-rich rocks in the RSJC mélanges, later refined in somewhat more detail by Hertwig (2014). Because the amount of jadeite can actually be quite variable, the term jadeite rock will be used as a general root term in this paper from here onward. In these studies it was recognized that two distinct suites of jadeite rocks can be differentiated. In one suite quartz is never found in the rock matrix, but albite is common. The second suite contains common quartz in the rock matrix. The first suite comprises jadeitite s.str. (i.e. > 90 vol.% jadeite), as well as jadeite-rich albite rocks, jadeite-rich omphacite rocks and rare pumpellyite-bearing jadeite-rich rocks. The amount of jadeite generally exceeds 50 vol.%. The quartz-bearing suite is very heterogeneous, varying from jadeitite s.str. to rocks containing jadeite, lawsonite and quartz in varying proportions. The quartz-bearing suite has also been observed to form concordant layers and discordant veins in blueschist, a very rare feature worldwide. Draper & Nagle (1991) already noted that the distribution of block types in the mélanges is not homogeneous, and Krebs et al. (2009, 2011) showed that the suite of HP metamorphic blocks in the group of mélange slices north of Loma de Catey in Fig. 1b differs from those found in the southern group. Whereas blueschists are present in both, eclogites are almost entirely restricted to the southern mélanges in Fig. 1b. This important difference is also reflected in the distribution of the two suites of jadeite rocks. Whereas examples of the (matrix)-quartz-free jadeite rocks are found throughout the mélange outcrops, members of the quartz-bearing suite are almost entirely restricted to the mélanges between Loma de Catey and the north coast (Fig. 1b).
Sampling and Analytical Methods
From the extensive sample suite of woodworking tools available from the Playa Grande excavation site, twelve axes or celts and one adze were chosen for further study. Thin-section analysis showed that nine of these represent jadeite-rich rock and two are blueschist. One
sample was a silicified green metavolcanic rock too fine-grained for further study and was not considered further. The data base from the RSJC with which these artefacts can be compared comprises over 100 different jadeite-rich rocks which have been observed in thin section, of which 67 have been studied in greater analytical detail. These data have already been presented in Schertl et al. (2012), Hertwig (2014) and Hertwig et al. (2015, 2016). Comparative data from the Cuban and Guatemalan source regions are available from GarcíaCasco et al. (2009, 2013), Cárdenas-Párraga et al. (2012), Harlow (1994), Seitz et al. (2001), Harlow et al. (2003, 2004, 2006, 2011) and Maresch et al. (2012). Mineral analyses on the artefacts were performed using a Cameca SX 50 electron microprobe with the following standards: pyrope (Si, Al, Mg), rutile (Ti), andradite glass (Ca, Fe), K-glass (K), jadeite (Na), spessartine (Mn), Cr 2 O 3 (Cr), Ba-silicate-glass (Ba). Operating conditions were 15 kV, 15nA; beam diameter ca. 3 μm. Cathodoluminescence images were carried out using a ‘‘hot cathode’’scanning-electron microscope of the type HCL-LM at Ruhr–University Bochum (operating conditions: 14 keV beam energy, ~9 μA/mm2 current beam density; for more details, see Schertl et al. 2004, 2005). Geochemical analyses were carried out by Acme Laboratories, Vancouver, Canada. Abbreviation of mineral names follows the suggestion of Whitney & Evans (2010). Trivalent iron was estimated for pyroxene and garnet by normalizing to 4 and 8 total cations, respectively. For amphibole, cations were normalized to 13, excluding Na, Ca and K. Total iron was assumed to be ferric in epidotegroup minerals, and ferrous in all others.
Petrography of artefact samples
Macrophotographs of the studied artefacts are shown in Fig. 2; microphotographs displaying typical features discussed in the following sections and obtained with the polarizing and the cathodoluminescence (CL) microscope are shown in Fig. 3 and 4. The observed mineral assemblages and estimated modal proportions are summarized in Table 1. Although the suite of artefact samples was randomly chosen from those expected on macroscopic inspection to contain jadeite, it is still surprising that seven of the nine jadeite-rich artefacts studied are in fact jadeitite s.str. with more than 90 vol.% jadeite. Omphacite in jadeite-rich artefacts is in general volumetrically subordinate and may be entirely absent (sample 31104); typically omphacite forms inclusion-free rims, often in needle-like sprays enveloping jadeite. None of the artefacts contain quartz as a matrix mineral and only two (samples 31105, 31108) contain rare inclusions of quartz within jadeite grains. Thus all are mineralogically analogous to the
(matrix)-quartz-free suite described by Schertl et al. (2012) and Hertwig (2014) from the RSJC serpentinite mélanges. Samples 31101 and 31104, on the other hand, are rich in lawsonite, with jadeite contents of 60-70 vol.%. Artefact samples 31110 and 31111 are, as determined macroscopically, glaucophane-rich blueschists.
Jadeitite s.str. Sample 31100, the first jadeitite s. str. in the suite, is fine-grained, mainly equigranular and without any preferred orientation of its mineral constituents. The jadeite grains are anhedral, and the interstices are filled by albite. Sodic pyroxenes in the matrix are zoned, showing cores of omphacite rich in inclusions surrounded by inclusion-poor jadeite. Some jadeite grains are rimmed by needles of omphacite. The sample contains small fractures and veins which are filled by euhedral clinozoisite with epidote rims, albite, greenish Mg-rich phlogopite and phengite surrounded by analcime. Needles of omphacite emanate from the matrix into the veins and are enclosed by the minerals making up the vein (Fig. 3a). The rock contains large grains of anhedral titanite up to 200 μm in size and zircon. Sample 31102 is another artefact lacking any preferred orientation. Jadeite is the dominant mineral, forms patches, is zoned and often has a cloudy appearance (Fig. 3b). Smaller flakes of phengite are evenly distributed within the matrix; albite is twinned and rare. Samples 31103 (Fig. 3c, d) and 31107 (Fig. 3e) also are nearly monomineralic jadeitite; 31103 is more coarse-grained than the latter and contains chlorite-rich patches. The fine-grained, in part rather dusty and cloudy looking matrix of sample 31107 consists of jadeite grains with an average grain-size of about 20-50 micrometers; it is traversed by net-like, structured areas of jadeite grains which reach 200-300 micrometers. In addition, thin albite-rich veins with omphacite needles occur. Sample 31106, another jadeitite s. str., shows no preferred orientation and is coarse-grained with large fibrous pyroxenes. It contains euhedral jadeite crystals overgrown by omphacite needles in crystallographic continuity and also poikiloblastic omphacite grains with inclusions of jadeite. Some jadeite aggregates are characterized by 120° triple junctions. Further constituents are phengite (Fig. 3 f, g), titanite, albite filling interstices, and analcime; zircon forms columnar crystals and often inclusions in jadeite, occasionally also in titanite. Samples 31105 and 31108 display an anisotropic fabric; quartz is present, but found exclusively as inclusions in jadeite and not in the rock matrix. Sample 31105 contains coarser-grained layers with “dusty” jadeite grains and also less “dusty” layers with elongated jadeite and titanite. Generally, the cores of clinopyroxene are more jadeite-rich and cloudy, whereas the rims are omphacite-rich and clear. These cores contain numerous inclusions of quartz, whereas the
omphacitic rims contain only very few such inclusions. Further constituents are euhedral lawsonite (Fig. 3h) and phengite, which are generally not aligned parallel to the schistosity. Sample 31108 is conspicuously foliated and similar to 31105, but contains more phengite, lacks lawsonite and is finer-grained. Clinopyroxene is elongated, forms a linear fabric and is zoned, with cloudy jadeite cores and clear omphacite rims (Fig. 4a). Isolated grains of omphacite can be observed. As in sample 31105, quartz is found exclusively as rounded inclusions in jadeite. Interstices are filled by albite; further constituents are phengite, prismatic epidote, elongated titanite and zircon.
Jadeite-lawsonite rock Sample 31101 differs from the above suite of jadeitite s.str. in that it contains significant amounts of lawsonite (Fig. 4b) and only 60 vol.% jadeite. The two minerals are clustered in alternating layers, giving the rock a schistose appearance. No quartz is found. It contains fracture-related (?) omphacite/albite-rich lenses (Fig. 4c). Lawsonite crystals define a distinct lineation within the layers. Jadeite is generally fine-grained; very rarely mm-sized zoned crystals can occur. The rock contains rare fine-grained phengite and patches of chlorite displaying “normal” grey interference colours. Occasionally, the chlorite is intergrown with calcite; titanite is accessory. Sample 31104 is a quartz-free jadeite-lawsonite granofels (Fig. 4d), similar to sample 31101 on the basis of the major minerals, but lacking any preferred orientation. It is cut by veins characterized by euhedral and prismatic jadeite crystals. It is significant that this rock, although the jadeite grains are slightly zoned, does not contain any omphacite. The other constituents are euhedral lawsonite as well as albite and fine-grained phengite, which fill the interstices between jadeite crystals. Epidote has a “cloudy” appearance and occasionally forms rims around jadeite. This is the only artefact in which paragonite was found, albeit only as one individual grain.
Garnet-lawsonite-omphacite-glaucophane schist Samples 31110 and 31111 are both glaucophane-rich schists. Sample 31110 (Fig. 4e) also contains chlorite but no jadeite, whereas sample 31111 contains jadeite but no chlorite. Sample 31110 is less intensely deformed and contains euhedral to subhedral poikiloblastic garnet. Albite is rare. Epidote and phengite form inclusions in garnet and lawsonite, but are also present in the rock matrix. Lawsonite occurs as large, elongate prismatic crystals. Titanite grains are abundant, small, rounded and slightly elongated and oriented parallel to the foliation. Sample 31111 contains higher amounts of generally fibrous glaucophane (often
rimmed or replaced by omphacite) as compared to sample 31110; omphacite and jadeite are slightly greenish in colour. Lawsonite forms fibrous aggregates and contains numerous tiny inclusions. Garnet forms euhedral fine-grained porphyroblasts (Fig. 4f); quartz is present as small grains in the matrix but also forms polycrystalline lenses. Minor constituents are apatite, albite, pyrite and other opaque ores.
Comparison with RSJC samples Figure 5 shows examples of typical features observed in jadeite- and glaucophanebearing rocks from the Río San Juan Complex for visual comparison with the artefacts studied here. The typical jadeitite s. str. shown in Fig. 5a is essentially identical with the artefact shown in Fig. 3e, although the grain size in the artefact is somewhat more variable than in the sample from the RSJC. The micrographs shown in Fig. 5b and d are almost identical in every aspect with those of the artefacts in Fig. 3c and d, where a late needle-like omphacite generation overgrows pure jadeite crystals. The interstices are mainly filled by albite. Omphacite overgrowth textures on jadeite are similar in Fig. 5c and Fig. 4a. Fig. 5e represents a jadeite-lawsonite rock (compare Fig. 4d) similar to the jadeite-lawsonite artefacts, although the RSJC sample contains low amounts of quartz in the matrix. Further such examples of jadeite-bearing rocks from the RSJC may be found in Schertl et al. (2012). Glaucophane- and omphacite-rich rocks (Fig. 5f) are quite common in the RSCJ (see Krebs et al. 2011); a thinsection image of a similar artefact (which does however contain additional lawsonite) is shown in Fig. 4e.
Cathodoluminescence (CL) microscopy
Numerous rock-forming minerals are sensitive to electron irradiation and are luminescent under the CL microscope. They can provide important information about their internal structure in this way (e.g., Pagel et al. 2000; Götze et al. 2013). This does not apply to minerals rich in iron, because iron is a quencher element. However, jadeite, the major constituent of the jade artefacts, is very sensitive to CL and thus this method can be used to document possible inhomogeneities such as crystal zonation (Schertl et al. 2004, 2012; Harlow et al. 2005; Maresch et al. 2012; Götze et al. 2013; Harlow et al. 2015). Such growth structures, which are often invisible under the polarizing microscope, can be used as “petrographical fingerprints”, and, in addition, as chemical pathfinders prior to electron microprobe measurements. An example is shown in Fig. 4g (sample 31107), which has also
been observed in other artefacts, such as sample 31102. In Fig. 4g the jadeite crystal in the center of the figure (which appears homogeneous under plane-polarized light and crossed polars) is characterized by a bright-green luminescent core (point 1) and a dark-green luminescent rim (point 2). This indicates that the darker rims are more iron-rich than the brighter cores (Table 2). Occasionally, the jadeite crystals of this sample can show oscillatory zoning. Omphacite (Fig. 4g, Table 2) shows only very faint luminescence, so that the needlelike crystals growing on jadeite are only indistinctly visible. Albite and analcime (Fig. 4g) are not luminescent. In some parts of the image, sample 31107 shows darker-green, irregular finegrained patches of jadeite where under plane-polarized light the jadeite has a dusty, cloudy appearance. Fig. 4h is another example of a CL-image for jade artefact sample 31103. Jadeite generally displays bright-green luminescent cores and dark-green rims, occasionally overgrown by weakly luminescent omphacite needles. The intergrowth microstructure of the jadeite crystals is much finer than in sample 31107 (Fig. 4g); in addition to the green luminescence colours, which are very typical for jadeites of the artefacts, bluish and reddish colours also occur. The sample also contains late calcite, which typically luminesces in orange colours. It can be shown that the CL-features observed in the artefacts have their counterparts in jadeitite samples from the RSJC Serpentinite mélanges. For comparison, the two CL images of Fig. 5g, h have been chosen as examples of jadeite-rich rocks from the Río San Juan Complex. The jadeite presented in Fig. 5g essentially displays the same features as the artefact in Fig. 4g. Greenish luminescence colours and oscillatory zoning are prominent. Jadeite crystals of Fig. 5h are similar to those of the artefact shown in Fig. 4h. They are finegrained, patchy and some crystals show bluish to reddish luminescence colours. Further CL features distinctive of jadeite-bearing rocks from the Río San Juan Complex have previously already been presented by Schertl et al. (2012; see their Fig. 5).
Mineral chemistry of the artefact samples
Representative mineral analyses provided by electron microprobe analysis are given in Tables 2 and 3.
Clinopyroxene Except for sample 31110, all artefacts contain jadeite compositions very close to the jadeite end-member (Fig. 6). As indicated in the previous section, the dark luminescent
regions of jadeite are characterized by higher amounts of iron compared to the light-green luminescent parts. This is documented in Fig. 4g, where the jadeite rim (point no. 2) contains a slightly higher amount of iron (Table 2, analysis 8) compared to the core (point no. 1; analysis 7 in Table 2). In general, the darker-green luminescent jadeite patches observed in sample 31110 are characterized by relatively high amounts of iron (e.g., Table 2, analysis 10; Fe 2 O 3 = 7.76 wt.%), leading to an aegirine content of ca. 20 mol%. For comparison, an omphacite analysis of the same artefact is given in Table 2 as analysis 9, which yields 3.53 wt.% Fe 2 O 3 and an aegirine content of 9 mol%. This difference mirrors the expected fractionation of trivalent iron into the clinopyroxene richer in Al. Additional jadeite and omphacite analyses of jadeitite artefacts 31100, 31102, and 31103 are given in Table 2. Analyses 11 and 12 are from sample 31111, a glaucophane-rich blueschist. In general, the clinopyroxene compositions of the artefacts and the miscibility gap between jadeite and omphacite mimic the data obtained for the (matrix)-quartz-free jadeite-rich rock suite of the Río San Juan Complex (Fig. 6, shaded area; see also Schertl et al. 2012; Hertwig 2014; Hertwig et al. 2016).
Plagioclase Plagioclase compositions are very close to albite end-member (> An 94 ). Two representative analyses from sample 31102 and 31103 are given in Table 3.
Epidote/clinozoisite The amount of Fe3+ in the epidote-group minerals of the jadeitite artefacts varies between 0.03 and 0.72 apfu. Two representative analyses from artefact sample 31103 are given in Table 3. Similarly variable compositions (Fe3+
0.09-0.52 )
are found in blueschist
artefact sample 31110.
Chlorite Chlorite in the jadeitite artefacts is Mg-dominant (Table 3) and contains low amounts of Mn (ca. 0.3 wt.% MnO). The tetrahedral site is occupied by 5.63-5.82 Si apfu (Al = 2.372.18 apfu). Chlorite from the blueschist artefact sample 31110 is higher in iron (Table 3); the tetrahedral site occupancy is similar to that of the jadeitites (Si 5.65-5.89 Al 2.35-2.11 ). White K-mica
In general, white K-micas are phengite with Si apfu ranging from 3.23 to 3.60. An example from sample 31102 is presented in Table 3, analysis 8. Harlow et al. (2006) and García-Casco et al. (2013) used a Si apdfu (atoms per double formula unit) versus (Mg + Fe (tot) + Mn) apdfu diagram for white K-mica to document K-mica compositions in Caribbean artefacts and potential source regions. For comparative reasons here and in subsequent discussions, we also use this diagram here (Fig. 7). The data points from the Playa Grande artefacts spread mainly along the trend of the Tschermak substitution (red double arrow), with some minor digression above this red line. The latter can be due to some solid solution towards tri-octahedral mica, but this effect should generally be less than 0.2 atoms pdfu on the Y-axis scale (e.g., Massonne & Szpurka 1997, and references therein). More important will be the effect of substitution of Fe3+ for Al[6]. It is significant that the very prominent Fe3+ substitution observed in some (matrix)-quartz-free samples of the RSJC (Hertwig, 2014), recognizable as one large and two subdued spikes to the upper left, is never seen in the artefacts. As discussed below, this may be a finger-print for pin-pointing source regions in the RSJC itself.
Paragonite The occurrence of paragonite in artefacts has been noted as a potential discriminator for source regions (e.g., Harlow et al. 2006; García-Casco et al. 2013), consequently it should be emphasized that this mineral was detected only in one single grain (Na 80 K 19 Ca 01 in the interlayer) of jadeite-lawsonite artefact 31104.
Glaucophane The compositions of glaucophane (Hawthorne et al. 2012) in the two blueschists (artefacts 31110, 31111) are similar (Table 3). They contain significant amounts of Ca; the Mg/(Mg+Fe2+)-ratio is between 0.55 and 0.65. The A[12]-site occupancy is less than 10%.
Garnet Garnet is exclusively found in the metabasic glaucophane-rich artefacts 31110 and 31111. It is in general almandine-dominant, but the spessartine component in the core may be quite high, up to 35.7 mol% in sample 31111, and decreases towards the rim. The almandinecomponent behaves antithetically. Compositions vary in the following ranges: sample 31110; Alm 40.9-59.1 Grs 24-31.3 Sps 1.8-20.7 Prp 3.2-11.9 And 1.3-2.8 Uv 0-0.3 ; sample 31111: Alm 32.4-42 Grs 26.6-28.3 Sps 25.8-35.7 Prp 2-2.5 And 0.3-1.8 Uv 0.1-0.2 .
Dark mica Present only in jadeitite sample 31100, dark mica is characterized by Mg/(Mg+Fe2+) between 0.79 and 0.82 as well as Al[6] in the range 0.35-0.37; it can be classified as phlogopite.
Analcime, lawsonite, calcite, titanite The analyses of analcime and lawsonite yield compositions nearly identical to the endmembers Na[AlSi 2 O 6 ]∙H 2 O and (CaAl 2 [(OH) 2 |Si 2 O 7 ]∙H 2 O, respectively, and are therefore not given in Table 3. Generally, calcite is also close to end-member composition. In rare cases (sample 31103), calcite may contain low amounts of iron (0.2 wt.% < FeO < 0.28 wt.%). Titanite contains low amounts of Al 2 O 3 (
