The Eocene-Oligocene Nanchititla dike swarm, eastern Michoacán, México

Figures & data

Figure 1. Schematic map showing the location of the Nanchititla dike swarm and the geological setting. (a) The Nanchititla dike swarm is located in the eastern Michoacán state. (b) The mapped dike swarm inside the red square is located at the western end of a regional lineament of Eocene-Oligocene volcanic centers and left lateral faulting indicated by NW-SE purple lines in the map (Morán-Zenteno et al., 2007). Shortening structures with N-S axis in grey lines appear at both sides of the studied area.

Figure 2. A view of the two main structures that limit the Nanchititla dike swam at the east and west side. (a) N-S section of the Tzitzio anticline in contact with the red beds that host the Eocene-Oligocene dikes. (b) A west to east view of Sierra de Nanchititla, constituted by a thick package of ignimbrites in contact with Oligocene lavas that rest unconformably on the red bed sequence.

Figure 3. Dike outcrop showing typical porphyritic textures with high plagioclase phenocrysts content. In (a) the dike-host rock contact is observed, the circle in blue enclosed a section of dike rock in situ that contains giant plagioclase phenocrysts. Photograph (b) is a close up of the area in the blue circle that shows individual plagioclase phenocrysts and glomeroporphyritic clots of plagioclase. (c) The groundmass shows an intersertal texture, here is shown a close view of plagioclase phenocryst twinning. (d) Rocks were classified according to the phenocrysts size distributions calculated with CSD Corrections from Higgins (2000). The calculation of Crystal Size Distribution uses geometric parameters of an ellipse (white line) that approximates to the 2D phenocrysts outlines (light blue line).

Table 1. A summary of geochronological data that includes new, recalculated and previously reported ages for Nanchititla dikes and other magmatic units.

New and compiled ^40Ar/^39Ar ages
Sample	Lithology or Formation	Lat N (UTM)	Long W (UTM)	Mineral	tp or (Wm) Ma ± 1σ	% ^39Ar	MSWD/n	tc Ma ± 1 σ	(^40Ar/^36Ar)i ± 1 σ	MSWD/n	Reference
MFD8	Mafic dike swarm	2092524	332078	Gms	34.50 ± 0.37	79.28	0.67/6	34.90 ± 0.90	292 ± 7	0.8/6	This work
				Pl	41.93 ± 1.66	59.28	0.1/3	41.24 ± 1.81	301 ± 2	0.5/7	This work
MFD49	Mafic dike swarm	2106110	327826	Gms	(35.69 ± 0.24)	88.56	3.65/9	34.57 ± 0.50	322 ± 10	3.4/11	This work
				Pl	–	–	–	34.47 ± 0.41	324 ± 8	3.1/12	This work
MFD78	Mafic dike swarm	2100938	340340	Gms	32.39 ± 1.07	62.5	0.58/4	32.30 ± 1.62	299 ± 6	1.5/17	This work
MFD92A	Mafic dike swarm	2103436	329799	Gms	33.95 ± 0.40	74.33	0.80/6	33.65 ± 1.33	300 ± 9	2.3/9	This work
				Pl	36.40 ± 0.85	42.38	0.62/3	32.88 ± 1.29	320 ± 4	0.04/4	This work
MFD92B	Mafic dike swarm	2103436	329799	Gms	30.45 ± 0.45	68.10	0.30/5	29.69 ± 0.73	304 ± 6	1.7/7	This work
				Pl	36.92 ± 0.58	79.6	0.40/5	35.96 ± 0.80	301 ± 2	0.3/6	This work
LS 42	Dike basaltic	2097820	329001	Gms	36.22 ± 0.94	67.49	0.66/3	35.02 ± 1.02	348 ± 12	2.56/8	Ages recalculated from Serrano-Durán (2005)
				Pl	(34.07 ± 0.79)		0.61/4	34.97 ± 4.30	288 ± 48	2.1/6	Ages recalculated from Serrano-Durán (2005)
BEJ 9	Dike andesitic-basaltic	2098470	338776	Hb	(36.26 ± 0.51)		0.5/7	36.8 ± 0.91	291 ± 6	0.4/7	Ages recalculated from Serrano-Durán (2005)
LS 3	Dike andesitic	2103686	334839	Wr	(38.17 ± 0.63)		0.66/5	32.80 ± 6.59	322 ± 32	0.57/5	Ages recalculated from Serrano-Durán (2005)
TC 05	Dike andesitic-basaltic	2078637	283720	Wr	37.59 ± 0.63	100	0.12/4	37.62 ± 0.86	295 ± 6	0.15/8	Ages recalculated from Serrano-Durán (2005)
TC 07	Dacite	2106369	312971	Wr	43.11 ± 1.37	100	0.67/4	45.04 ± 2.86	260 ± 6	0.34/8	Ages recalculated from Serrano-Durán (2005)
LS 84	Tuzantla dike o microgabbro	2120453	333204	Pl	24.55 ± 1.55	78.69	1.2/4	24.81 ± 2.56	306 ± 6	3.16/10	Ages recalculated from Serrano-Durán (2005)
TC 01	Andesite	2087495	365554	Wr	51.70 ± 0.61	43.43	0.35/3	43.40 ± 2.80	714 ± 99	1.7/7	Ages recalculated from Gonzáles-Cervantes (2007)
TC 02	Nanchititla ignimbrite	2088341	362363	Sa	(36.50 ± 0.44)		0.84/6	36.23 ± 0.52	304 ± 10	0.89/6	Ages recalculated from Gonzáles-Cervantes (2007)
TC 04	Diorite hypabisal	2090072	349130	Gms	36.74 ± 0.19	93.43	0.20/4	36.65 ± 0.35	301 ± 22	0.26/4	Ages recalculated from Gonzáles-Cervantes (2007)
NAN 55	Syenogranite	2087747	350016	Fsp	36.10 ± 0.16	95.47	2.49/13	36.29 ± 0.25	283 ± 11	2.9/13	Ages recalculated from Gonzáles-Cervantes (2007)
NAN 64	Andesitic basaltic	2087792	343650	Wr	37.57 ± 0.22	84.55	0.90/4	36.79 ± 0.44	340 ± 18	1.3/5	Ages recalculated from Gonzáles-Cervantes (2007)
NAN 68	Andesitic basaltic	2087378	348934	Wr	35.65 ± 0.33	25.05	0.69/2	–	–	–	Ages recalculated from Gonzáles-Cervantes (2007)
GO 05	Ignimbrite	2080149	397304	Sa	35.18 ± 0.45	79.95	3.74/4	35.15 ± 0.44	310 ± 36	3.3/6	Ages from the Sultepec-Goleta volcanic succession presented by Díaz-Bravo and Morán-Zenteno (2011)
GO 372-07	Ignimbrite	2064140	386008	Sa	36.17 ± 0.55	96.40	1.55/6	36.61 ± 0.51	60 ± 54	1.3/7	Ages from the Sultepec-Goleta volcanic succession presented by Díaz-Bravo and Morán-Zenteno (2011)
M 29-9-1	Dacite	2032163	320058	Pl	43.23 ± 0.45	85.68	1.5/4	43.64 ± 1.71	293 ± 11	2.0/4	Ages from the Huetamo-Ciudad Altamirano region presented by Martini et al. (2009)
M 15-2-4	Granodiorite	2023872	265106	Fsp	45.59 ± 0.39	82.26	1.2/4	45.72 ± 0.30	302 ± 4	2.8/18	Ages from the Huetamo-Ciudad Altamirano region presented by Martini et al. (2009)
M 15-2-3	Ignimbrite	2024356	262022	Pl	38.37 ± 1.03	40.57	0.3/3	37.21 ± 0.56	311 ± 6	1.64/11	Ages from the Huetamo-Ciudad Altamirano region presented by Martini et al. (2009)
TUZ-01	Tuzantla mafic dike	2128005	338125	Wr (?)	30.56 ± 0.43	54	1.84/4	28.8 ± 0.6	306 ± 3	1.17/7	Ages from Gomes-Vasconcelos et al., (2015)
Compilation of K-Ar ages
	Lithology or Formation	Lat N (UTM)	Long W (UTM)	Mineral	%K ± 2 σ	^40Ar* nl/g	%^40Ar*	Age Ma ± 2 σ	Reference
LS 84	Tuzantla dike o microgabbro	2120453	333204	Pl	0.20	0.268	45.9	34.8 ± 1.1	Ages recalculated from Serrano-Durán (2005)
GO 01-04	Sub-volcanic ignimbrite dike	2068045	386060	Sa	9.64	6.181 × 10^−10	96.9	36.60 ± 1.10	Age from Díaz-Bravo and Morán-Zenteno (2011)

NOTE: & Step heating measured in an MS-10 spectrometer, $ Step heating measured in a VGA spectrometer. Errors are reported at 1σ level; ti – integrated age; tp – plateau age; tm weighted mean age; tc – isochron age. †Weighted mean of one-step fusion experiments. Bold indicates preferred age.

Figure 4. ⁴⁰Ar-³⁹Ar age spectra, preferred age and inverse correlation diagram; respectively, for samples: (a and b) MFD 8, (c and d) MFD 49, (e and f) MFD 78, (g and h) MFD 92A, (i and j) MFD 92B. Geochronologic data and sample location are reported in Table 1. When available duplicate experiments on different mineral phases are indicated. Dating was performed at the Laboratorio de Geocronología of CICESE (Centro de Investigación Científica y Educación Superior de Ensenada Baja California). Acronyms: t_p, plateau age; t_c, inverse correlation diagram age.

Figure 5. Distribution of dike orientation and thickness in the Nanchititla dike swarm (a and b). Some dikes have planar and tabular geometries as shown in (c). Photograph (d), (e) and (f) are examples of finger-like dike intrusions are a single segment or several segments formed from the main dike intrusion. Dike-sill systems were also observed at different levels of the host rock as shown in incise (g) and (h).

Figure 6. Rock samples that show the contrasting plagioclase phenocrysts size content in Nanchititla dikes. N, M, G and Nv classification is based on the results of Cristal Size Distribution Analysis made for these rocks using the method of Higgins (2000). Nv is aphanitic texture Most of the dikes have porphyritic textures being the N type (Pl < 0.5 cm) the most abundant.

Figure 7. Percentage of dikes with G, M, N and NV type rocks in different range of dike thickness. The percentage was normalized to the maximum number of dikes observed in each range.

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